For the Sense Motive thing, just create a Flaw called "Gullible" that imposes a -10 penalty to Sense Motive checks. Given what you're trying to do, very few DMs should have an issue with that. There's an alternative class feature from Arcana Unearthed that replaces the Monk speed bonus with Barbarian damage reduction:
Durable (UA, p 58): Gain damage reduction as barbarian. Lose speed enhancement bonus and bonus to AC (but not wisdom to AC).Overall, it's a pretty bad trade, but it seems to work for what you're suggesting. If you're worried about the loss of speed, you could always take the Quick trait (-1 hp per level, +10 to your movement speed). Shou Disciple is generally a good choice for a monk, since it increases your versatility and bab. If you want to go the natural armor route instead of (or in addition to) switching out for damage reduction, there's always the Improved Natural Attack feat. It won't help your char be more like Ranma, but it will help him/her deal a shit-load more damage per hit. I wouldn't do that, though...what you were talking about is definitely DR rather than natural armor.
Greater Curse is probably the best way to handle the gender-switching and the cat thing. In the latter case, he/she becomes panicked around cats (within X-feet proximity), and if he/she cannot escape within X-rounds, he/she becomes enraged (as the Barbarian ability, or maybe just a bonus to strength or damage, depending on how it works in the comics).
A lot of people dislike monks, but I think the class is the best way to represent a kung-fu flavored martial artist without too many supernatural, over-the-top abilities (i.e. ToB)...look through this thread for more ideas on customization:
http://forums.gleemax.com/showthread.php?t=967118 Maybe just go straight monk, or monk/shou disciple...multiclass a little or look at the thread I posted if you don't like the flavor of the monk's supernatural abilities.
I'm not actually familiar with the Ranma character beyond what I've read in the Wikipedia article, so I think that's the best I can suggest.
Edit: Actually, Monk 16/Shou Disciple 4 with the UA alternative class feature seems like a good, simple build for this...you get 16 bab, flurry with light martial weapons, 2 bonus feats, none of the REALLY supernatural abilities from the monk class, plus light armor and the Shou Disciple dodge bonus to make up for the loss of your monk AC bonus.