Building things yourself saves very little money, go and have a look at the amount ive spent building my mini engine up myself for an exmaple (its a very similar engine to a cossie and almost all the parts would be the same price for a 600bhp cossie motor as they are on my 2.0 16v vauxhall engine)

The labour cost is tiny really compared to the parts cost.

you are talking 2 grand out of 15 probably that you are going to save at most, and often its MORE expensive doing it yourself as you end up sourcing individual parts that cost more than when a tuner buys them in bulk.