err..... how long is a piece of string?
depends how you intend to do it!
Using new, reconditioned or 2nd hand parts, genuine or pattern parts?
Doing everything yourself or paying someone else?
All standard, or slected "improvements / enhancements?
I think someone on here worked out a figure of about £20k using all new parts and doing the work themselves, but you could probably do it for half that if you used 2nd hand / reconditioned bits.
Many people would use a 200 or 300 TDi rather than a TD5 because they're simpler and more reliable.
The sky's the limit as far as enhancements are concerned.