A v6 is a proper man's motor, lad... My mechanic always quotes me £500 plus parts and labour..
Even if you take a good engine and just dismantle it and put it back together again it'll touch you for £400:
Belt tensioner is circa £90
Water pump is about £70..
Head gaskets @ £80pr (if you can find them.. otherwise use gaskets from "old" GTV6 2.5)
Rocker gaskets @ £30 (genuine parts, moosh!)
Cambelt @ £30 (as above)
Plug gaskets @ £20
Oil @ £25 10W40
Filter @ £10
Plugs @ £25
various fasteners, O-rings and sundries @ £20
If you have any bent/worn valves, rings, pistons, journals, rods or bearings.. just "guestimate" them at £40 each..
If you want to strip down and rebuild an engine, I'd start with a Fiat Cinquecento (FIRE) engine basically (just 4 of everything and a lot easier to wrestle with).