I believe that 12.2T was the earliest release that supported IPv6. If 2500 series support 12.2T you may be in luck but that hardware has been unsupported for well over a decade. Even if you do find a release that supports it you may have flash and memory constraints.
Yea, I'm not sure if the 2500-IPv6 path is a dead-end or not. I have 1 2514 with 16MB of memory and flash, and it won't load an IOS that might have v6. Won't even tftp it.
I didn't knwo that the 2500 series could have more than 16. Can they have 32MB ?
Too old, too old-school, too real-school for all that. I buy a router and it's mine, i dont' pay every month for it. rentals are good for the new kids, kids that don't want to get their hands dirty opening a router, kids that don't understand how they're throwing away money evry month, kids that want it NOW but don't want to buy $20 for a router once, but will pay $4.99 every month for years. Virtual is not real, it's not what I'd hire someone to be able to do.
VIRL, or GNS3, will run IOS as it runs in a physical device. They are EMULATORS, not SIMULATORS. A simulator provides some canned exercises to practice specific tasks. But running a device in emulation will give you a fully functional device that can be used to configure and test any feature supported in the IOS version.