If you are changing the whole drive train dont buy a whole bike because you will be paying big for a bunch of stuff you dont want and wont use. Many people buy beautiful frames but are so shocked at the cost of high end parts that do the frame justice, the frame sits in the garage for twenty years.Good parts are expensive so I look for them first.Secure the parts you want , dont settle for less. Especially wheels...I always start with the wheels because they are the most expensive thing and hardest parts to find / build.In fact gather most of your parts first then look for a frame that way you wont be scrimping and trying to save money getting your build together in the end .If a frame has rust on the outside it will be double on the inside .Dont buy a rusty frame, there are thousands of frames. I think you will discover finding a great frame is the easy part.