In addition to two Netpro's comment. You also need to know which feature set is required. And what specific feature you require. With the limited DRAM (2M) and unknow flash size. You may able to upgrade to 11.x only. Please more.
This maybe hopeless as the oldest IOS I see on CCO and via ftp (ftp-sj.cisco.com) is IOS 11.2. IOS 10.3, 11.0 and 11.1 are there but the only image available was for the 7500 in IOS 11.1. Also I didn't see any 2500 images that would run with less than 4 megs of RAM but I didn't look at every feature set just IP, IP Plus, Remote Access Server, etc.
It's easy to upgrade the RAM (DRAM) on the 2500's as a 16 meg stick of memory should only cost about $5 off of eBay.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...