I have 15 Cisco 2500 routers for my lab, bought them all on Ebay.
I have them all mounted in a "shock proof" rolling 2 sided "DJ" 24u tall audio rack. If you have ever been to Cisco Networkers - it is how they carry lots of the lab setups for the different sessions.
Shop around can get some GREAT prices.
Also do a search under routers/Cisco for "labs" to see the groupings of routers, switches, cables etc. for an instant lab.
For CCNA 2 2501's and 1 2502 and 1 or 2 switches is all you really need. 2502 is a token ring router but it still has 2 serial interfaces that can be used to talk to the other 2 routers.
CCNP and CCIE - the more the better so you can create all sorts of arrangements and configurations.
E.A. Broda
CCNA, CCDA, CCAI, Network +