You could get the 2610XM's or 2611XM's with max ram and flash (256MB ram and 48MB Flash). They support the most recent version of 12.4T and support ipv6 and have enough ram and flash to run SDM, call manager express etc. You will need the newer Gen1 ISR/Gen2 routers for anything .15 or newer. Keep in mind the licensing has changed coming from 12.? to IOS 15. With 15 all features are licensed based now. Meaning you can buy a base license for IOS 15 but if you want a security features set that would require a license etc. For a newcomer I say the 2600XM's are a good place to start. They run around $150.00 used. If you want better than the 1800's are your next best bet. May want to check out this link for used equipment.
http://www.certificationkits.com