Hi Mickey welcome to the forum. To answer a few of your questions (just my opinion )first
1)Need to go and buy a bike first, have had them in the past but never sure if they were the right size for me? Can anyone tell me what size bike I should buy, think I am 5'7" and a bit.
A lot of bike frames differ in size so the general consensus is TRY before YOU BUY. Having said that at 5`7ish you are probably around a small/med in that order. Is it xc you want or more do it all. Some xc bikes are more race oriented than others, ie longer top tubes and more forward riding position.
2)As for shoes, that`s a personal thing some feel more connected to the bike by wearing clip in shoes/pedals whilst other`s like the freedom of flat pedals and the knowing that if all goes bad you wont be glued to the bike lol. If you want the special shoes/pedals depending on size there might be some going cheap as Macca just broke his ankle (off work 2 monthsish) because his foot didnt disconnect from the pedal.
EDIT- Have I helped you decide yet
We have a regular on here Gturn aka (Luke) from Triton Cycles,which is a friendly shop that has served me well over the past 5-6 years, you could give him a call and see where that takes yer. Tell him you are from the forum of course.
http://www.tritoncycles.co.uk/My mate just bought
this from triton only last friday.