Never bought one to be honest mate but the actual engine is basically the same (block, head, crank, pistons, rods).

If you can find one you only need all the bolt on bits like manifolds etc. You will need a sump (not the easiest job but do able if you have the right tool as the rear 2 bolts are a cunt to get at).

This company actually converts the 2.5 DI Transit to turbo if it's of any interest...I am assuming the conversion can then be optimised a little bit more than what they quote

http://www.tb-turbo.co.uk/vans.htm