Taking your questions in no particular order:
Look in the back of magazines or club notice boards for shares.
Your Katana/warrior syndicate sounds great. No capital ususally means no bills!!! Remember in most syndicates the definition of the limit of your liability is what you put in. If you are in a syndicate and a big bill arrives you will be expected to chip in your share, or forfeit your share. Look on the bright side , you only get to see 1/4th or 1/10th or 1/16th of the bill, an absolute bargain!! We needed a new engine once, it cost us £700 each, not too bad.
White Waltham has lots of good shares. Theres a couple of 16 man cherokees there that run very well.
I have never heard of a group that has availability problems. Most have utility problems. The more they fly the cheaper it is. School planes do an average of 600 hours per year, most syndicate planes struggle to hit 200.
If you want to do an IMC use a school plane. Syndicate planes do not usually pay attention to keeping the IMC instruments working.
As for budget £15,000 is way too much. You'll end up with a share that is very hard to sell. I would start with a £2000 ish share, get your 100 hours ish, sell the share, do the IMC then buy a share in a Yak52 (again lots at WW) and do aeros!!! Then either give up flying or become an instructor/airline pilot.
Eirther way , with your attitude you will get a lot of flying and have a lot of fun.