I ride my kx everywhere, dunes, woods, tracks, and fireroads. It is great everywhere. It can be hard to ride in tight stuff, especially if you are new. If you have big hill climbs or whatnot without a great run-up, it will be hard to get up the hill. There is not a lot of bottom end like a kdx, and the top end is where the power is made.
The kdx can be louder than the kx depending on the exhaust system. If it is stock to stock, the kdx will be louder, but deeper. The kx sounds a bit like a loud chainsaw.
You will not fit the kx most likely at first. You may want to look into shaving the seat and/or purchasing a kouba link which will lower the bike an inch or so. The kdx is quite a bit taller than the kx too, so if you fit on your friends, you will probably fit a kx. It all depends on your build.
You may be better off with one of the crfs to learn on. Once you get comfortable, maybe upgrade to a bigger better bike.
By maitenance, I mean, changing the oil more often, rebuilding the top end or engine yearly (if you ride a lot). If you follow the manual, there is just a lot of stuff to do.
I think that you would be better off with the crf :)