For whether or not it's okay to run it fast, I advise you to use the search function. In short, it should be fine at 55 for awhile.
For new or used, depends on the bike. Pre 2001 has a different carb, drum front brake, and a kickstart/electric start. 2001 and newer get a disc brake, different carb, and lose the kickstart (however you can put one on).
For $2500 you should be able to find a TW less than 10 years old in good shape, in most places. Good luck.