Hello, there are loads of mods that can be done to these bikes and quite a few places to buy parts, the sky and the depth of your pockets being the limit.
As for things to watch out for will be similar to other 18 year old bikes. Just ride it and keep it in good fettle and you won't go far wrong.
As for the height, you can get longer dog bones to raise the rear a bit but to get it a lot taller you will need to change the forks and rear shock.
You could if you really wish change quite a few bits and make yourself a Paris Dakar racer.