This is just for the persons information i haven't got anything to do with the sale.
http://cgi.ebay.co.uk/Mini-B-300-EPI...QQcmdZViewItem
Seems a reasonable price. We have this type in our rib. Quite sturdy and easy to use.
I really don't think that a raft is going to be of much use to you either its going to be a complete pain in the bum to have it anywhere that you can get the thing out quickly if required.
On ebay as well there are plenty of diving Dry suits for sale at under 100 quid.
Or am sure you could rent a dry suit for a couple of days for 30 quid.
Don't go for the thin type. Go for a neoprene thick one. They turn into wet suits if they leak and still retain some bouyancy (although not much).
Personally I think the beacon is a good idea for that price but I wouldn't bother with the rest of the kit apart from the life jackets. Persoanlly the accident statistic for ditchings in UK waters and G reg aircraft, one in the last 3 years at Dundee, and one up in my old school 5 years ago ish, are good enough that its inside my not worth it zone. Single engine IMC has very similar risks if not more so but it doesn't seem to generate as much debate.