So after modding a WRT54GL for a job at work I decided that I would do the same for my own one but add as many hacks as I can. This is bit of a nostalgia trip for me as I still have some linksys routers from about 10 years and remember liking them then. They've proved popular in the community as being very hackable due to running linux.
After much searching I managed to find a WRT54GS V1.1 on ebay and picked it up for about £30, the reason I specifically went for this version is it's one of few that come with 32mb ram and 8mb flash. A comparison of different models can be found here