3rd quote is bloody good mate... if id have known about them last year id have sooner had a full new lump in mine instead of fookin about like i did and spending loads!!
id do a few more checks that all is ok with the firm that supplied that quote and there deffo sure they know what car youve got and havent got you confused with a different golf (ie 8v etc)
the vw quote aint bad if thats for a full engine, i got quoted at around £1200 just for a new 20v head with valves/guides last year

second hand engine is expensive just for a lump, id be wanting the full lot, manifolds, pipes all other bits aswell for that, but then if its come out of a smashed car, youd really need some proof that it wasnt involved in the action!!
what ever your choice mate i hope it all works out, quote 3 seems the best to me provided they know what there doing..20v engines are a f@cker to set up!!!!!