Hello Luis,
If you do not need TTL flash a 501CM is a good choice.
If you think TTL flash may be usefull for you any 503 body will right.
In short what you can expect from the later models:
503 CX TTL flash (basicly a 500CM with TTL flash)
503 CXi like 503CX but also takes a winder, has Palpas coating inside for improved contrast.
503 CW like 503CXi but has the larger mirror that does not vignet the image on the focussing screen.
The 501CM is a 503CW without the TTL flash option.
The 501CM also has the larger mirror with the Gliding Mirror System.
The 501CM and the 503CW are the best options.
They are later models of which the 503CW is still in production.
KEH in Atlanta is a reliable source for used equipment.
Their rating system is very conservative.
Keep that in mind when you compare prices.
www.keh.com
A brand new 501CM will serve you several decades if you treat your camera nicely.
The price is about right which does not mean you should not not try to knock something of or get a free lensshade with the deal.
Buying on Evilbay takes some experience and a lot of luck.
A reliable dealer will give you better service and usually guarantees their used equipment.
Do not forget to mention what you have decided. Nice to know.
Paul