Hello & welcome looks very nice,
I'll try and point you in the right direction with regards to the questions,
1, sounds like the common shock issue
2, the clacking noise could be one of many things, possibly drive train movement, they are a bit slack, maybe investagate the various bushes,
3, at what speed and revs does this sound occurr? The injectors are noisy and do tap away plus the engine is know to be a bit noisy, unless it's horrible knocking sound it should be ok, has the car been mapped for UK fuel yet ?
4, Is it the factory sub in the side of boot ? An aftermarket head unit will have pre outs for a sub to be wired in so you should be able to keep it
5, the ariel is usually the bigger back cable with a 3.5mm jack on the end
6, I guess the car has been sat on the boat for a while so it may just need driving more, or some more grease on the back of the pads
7, it's probably got some fluff in or general grime, you should be able to take the b pillar trim of to get at the reel and give it a bit of a clean
8, not sure about the steering, mines imported too but it doesn't feel that light, what pressure are you running in the tyres ?
9, I've sat in newage sti's and didn't notice the seats to be small they do grip you tightly though, may be worth seeing if any impreza owners would measure the base dimensions?
Anyway hope this helps and I'm sure someone else will be along to offer some more help