If money is not an issue for you then it really all depends on which model you like, the 2002 mustang
or 2005+ mustang. You can mod any car you like, but you cant change its basic frame. If you know what you want then go for it!
Now if money is an issue for you then I would stick with modding your 02. $2000 is enough to get yourself some decent rims and enough left over for aftermarket lights, fenders, etc. But if you want a decent paint job then expect to save up another grand! (if you buy a new mustang say 25,000 you gonna be paying 300 a month for like 7 years) thats 300$ to mods on you 02 every month!