Ash he won't find a yz250x anywhere close to 5k, the least he would spend is 7k if he finds one. And to be honest, I own a YZX and a regular YZ, if he purchases a decent shape YZ250 for about 3500, put on a skid, plate, radguards, a stand and softens his suspension he will be pretty close to the X with the exception of the gearing ratio and the 18 inch wheel instead of the 19.
OP, Trials and Wingboy are most likely right, starting on a trials bike will get you up the learning curve a lot faster than a motocross bike, however if you don't wanna go that route, I would definitely recommend a YZ, parts are really cheap and since the bike and engine has been fairly the same since 2002 (or even 1999) there are plenty of them out there. You can go really slow on a yz250.... you just have to follow me and I'll prove it lol.
You also have the option to learn on something smaller like a 220/230, those bikes are great and on the cheap end for woods riding and once you learn how to ride on them and know what you don't know now, you will be in a better position to choose your long term bike/Style.