Before I answer, it's been a good year or so since I've read the first series, and I'm up to book 2 of the second arc. Just wanted you to know what my qualifications were for answering this:
I've heard the nudity is similar to that in Sailor Moon. Is this true on all occasions?
Absolutely. I think there are only 4 shots of it in total, and all of them are transforming magical girl shots, aka Barbie style nudity. Nothing to worry about here.
Also, is there any homosexuality in it? That's the one thing I can't stand in anime or manga...(Kamatari was an antagonist, so he doesn't really count! XD)
In MKR II, there are two characters whom I suppose you could stretch and say could be more than friends (Eagle and Geo, I believe are their names), but no, nothing outright. It's like the majority of manga today--two boys happen to be good friends and the rabid yaoi loving fangirl base proclaims they are gay. No, nothing outright.
How much sexual content is there?
Very, very little. As mentioned, the transform scenes. And the plot of the first (and a wee bit of the second) is around the pillar, Princess Emeraude, and her forbidden lover. But all you see is them holding hands; I think there might be a kiss or two in the second but no one tramples off to bed or anything. The other couples are even less than that. There MAY have been one or two off-colored comments, but I really don't remember any...just covering all my bases here. Beware, though, Tantra, Tarta, Alcyione, and Presea do dress rather skimpily, but I've seen worse in a fighting game.
I'm not bothered by fantasy magic unless it starts getting occultic. Does that ever happen in this series?
No. The worse it ever gets is that the Magic Knight's mecha are referred to as "mashin" or "gods", and they speak to the girls. Guru Cleft is the one who teaches them their powers, and one of the villians is able to summon monsters. However, it is never occultic or sacrificial or anything like that. It's less, I'd say, than even RPG games like Final Fantasy.
Overally, I really enjoyed this series tremendously and I'd love to add it to my library as soon as I can. I think you'll love it!
Feel free to ask me if there's anything else I can do for you.