1. Were the Ranma and Harry Potter universes the same for the story than Jusenkyo would likely have Repello Muggletum the muggle prevention charm on it. Jusendo would be being made into a tourist resort for the magic community. China and Japan would have there own magical communities that likely would not do things the same as the UK one. It would likely be that China and Japan have a lot more trouble and less inclination to follow the International Statute of Wizarding Secrecy to the degree that they are supposed to. The Asians don't have to hide as much since Asians are in general far more accepting of magic than the western world is. While the Wizards in Harry Potter use wands that does not mean that every magical community does so it is possible that other communities use different power focuses.
2. While both series have ghosts there are differences between the two series. The ghosts and spirits of Ranma 1/2 have a lot more abilities than those in Harry Potter. They can possess people, go invisible, create spirit fire, interact with objects and people to a much larger degree, place curses, and transfigure things (paper into underwear for example or Shampoo into a cat though in that case it required more time and special circumstances). The beings that were always spirits and not ghosts (of which Peeves is the only one shown in Harry Potter and that was told in interviews who can interact with objects) in Ranma 1/2 can do what the ghost can but at least one showed the ability to create shields. Ranma has a lot more ghosts and spirits/kami than Harry Potter did. Some of the objects in the Ranma manga seem to be powered by the spirits/kami and not by magic such as the spirit wards.
3. Magic in Harry Potter was mostly point wand and say made up word/s. There were no magic wands in Ranma. Both series had potions (Ranma also had incense which is similar to potions just burned after gathering the ingredients, pills, and powders), magical creatures (though the creatures were different), magical items (the most common type of magic in Ranma 1/2), magical plants (some different but some were the same in both like Mandrake root), divination/prophecies (the reason that the calendar was shown was because Ryoga was doing a type of Japanese numerology called rokuyo to figure out it was a good day and the sakura mochi were supposed to tell you if you were destined to be with someone though my take on that was that it showed if you were compatible not destined), and probably more. Ranma has martial arts though which does allow many similar things to magic like vacuum blades, ki blasts, pressure points with various effects, etc.
4. Technology. Dudley had a Playstation before they even came out in Japan, flying car, train, a bus, magic cameras, magical radio, and the use of a telephone like once is pretty much the extent of technology used in Harry Potter. In Ranma pretty much every bit of technology you would expect to find in a Japanese house in the mid 1990's was used (karaoke machines, rice cookers, microwaves, tvs, vcrs, video game systems, water heaters, cameras, video cameras, floor buffer, telephones -even calls to China, etc.) Several means of transport including trains, buses, boats, hot air balloon, and helicopters (planes were shown like in the first of Ranma's nightmares seen and flying over Hong Kong but not ridden in the series). A few bits of nonexistent to the extent shown in the real world tech - power armor, jet pack, and memory metal. Most of that doesn't matter for the crossover except several characters including Ranma would and did use technology in combat. It is said by Hermione that electricity doesn't work at Hogwarts (and only Hogwarts there is no reason to think it wouldn't work elsewhere and the book she read it in could have been wrong if you really want to have it there) not counting electric weapons (Kodachi's stun baton, collar for her pet, electric trap that shocked Akane, etc.) that still leaves all the other non magic and non electricity using stuff used (tear gas, gas masks, knockout powder, various weapons like swords, staves, knives, explosives, sleeping powder, chloroform, smoke bombs, etc. - guns were shown but never used by one of the non background characters).
5. Since I am not the original author I would probably change the characters, mostly subconsciously. Generally I would try to keep characters in character but with this series I just can't see myself doing that. Hermione would discover that books are wrong on occasion, some are filled with lies, contradicting information, misinformation, etc. she'd also be bored out of her mind in class while still capable of answering all the questions asked she'd no longer care to do so. Ron would be less jealous, Harry would be less trusting and more of a leader. While character development would be included I would likely change the characterization for no adequately explained reason.
6. The entire house point system is screwed up and very stupid. The way I see it Ranma will ignore it entirely. There is no tangible reward for your house getting the most points. The way to get the most points is to be teachers pets. It directly pits the various houses against one another and indirectly pits the members of the house against one another (because it ostracizes those who lose more points). Ranma doesn't much care for following rules/laws and for the most part treats people the way they treat him. He will show respect to those that do so to him. While he would break the rules he will if caught accept the punishments given if he believes they are just.
7. What house would Ranma be sorted into? I don't think it really matters particularly since the person being sorted seems to have the ability to choose the house they want. It could also be argued that he would fit in any of the houses since he has traits that fit all of them but doesn't fit any of them perfectly (also it depends on what definitions of the various traits that are mentioned because several of them have more than one meaning of which Ranma may not fit the most common meaning but does fit some of the other meanings).
8. Japan and England have different schooling systems one thing in common to both is by the age of 16 school is no longer compulsory. The holidays, breaks and length of breaks, starting time (sept first for UK and in the spring for Japan), way the classes are set up, length of the school day (Japan was longer particularly back in the early 90's when Saturday was a school day as well), etc. all differ. Just pointing this out because the changes and less class time would probably effect Ranma somewhat. Though there is no real reason as to why Ranma would have to go to Hogwarts he could interact with things/people elsewhere and acquire knowledge of magic in other ways (such as hiring private tutors - the wizarding world like the real world runs on money and Ranma has the ability though lacks the inclination to acquire a significant amount).
9. Legilimency and Ranma. First off what is legilimency? It is not mind reading this has been said (or so I have been told) by the author. What it is, is viewing memories. Legilimency also doesn't work perfectly Harry was capable of keeping a memory from Snape even when Snape cast the spell out-loud with his wand. I don't think it would work too well on Ranma since he does have practice meditating and knows how to via soul of ice suppress emotions, and Ranma's mind would probably be a very strange and confusing place for the average wizard.
10. Innuendo and Puns. The Harry Potter books have quite a bit of innuendo (not that I caught most of it until it was pointed out to me) and the Ranma manga is chock full of puns (a lot of them were lost in translation or when still kept didn't come out all that great). Put them in a fan fic if you want but personally I don't think that it is easy to do while still keeping the fic interesting.