i find that all the mini sites are very bitchy. there are a couple of vendors that all they do is try turn everyone against the other vendors. most of the members are pretty clueless as well, i often read things like "oh you can't do that because no one else has done it before" just general stupidity like that. websites to try though would be north american motoring, mini2, and mini underground. i like mini underground the most i think. mini-mods.com.au is an australian site, but the traffic is so low there it is often not even worth going. if you want to buy into some of the vendor bitchiness, revolution mini works has a forum, but you basically can't talk about any product other than an rmw product. i also hate all the advertising mini2 and nam have.
i'm finding its harder to find parts for the older minis (mine is also 2003) as the aftermarket is moving to only supporting the R56 onwards.
LCA bushes, i ordered powerflex and got them in from mini mania in the usa. i couldn't find them anywhere else at the time.
i'm not 100% the msd coil was worth it. i had the cam and pulley installed at the same time and performance wise, they were the best bang for buck out of any of the mods. the 15% pulley takes the mini from 9psi to 15-16psi. at the time i only had the cai and msd coil installed. i had the coilovers already installed at that time as well, but i'm not sure i'd say they were bang for buck. big improvement though. moving to non-run flat tyres seemed to be pretty noticeable as well.
beyond the pulley, from what i've read headwork or an aftermarket supercharger is the next big step if you want lots more power, other than tuning of course. i'm a little skeptical, but i've seen dyno maps for minis with the same/similar mods as mine + a tune pulling 150kw+ on the dyno. american dynos must read high.
learning to tune the mini right now actually. i've made a few changes to my mini already with good results. i'm intending to offer a tune service for minis to help put some money toward my upcoming wedding. i think though the aftermarket for the mini as a whole is dwindling at the moment though, so i've probably learnt too late. i desperately need larger injectors though, which is why i'm learning to tune the mini regardless. save me some money in the long run.