You can do IS-IS on a 2500 so long as you have something like a 12.2 Service Provider IOS.

It still fits within the 16MB of flash (just)

Damned thing is, you need to then go back to a 12.3 Enterprise to do much of the other stuff, or have access to features you get used to like, DO SHOW RUN etc etc.

The old 642-801 which is extremely close to the 642-901 practicals in CCNP Self-Study BSCI 2nd Edition can all be done on 2500's and you can build this network with them:

Just getting that picture to work at all is a good exercise in itself. :)

Thanks for all your helpful advice though Wendell

Cheers
Shaun