The VR6 lump will be cheaper and if you find an entire Mk3 VR6 you'd be able to get all the bits. They're not expensive, I've seen VRs go for less than a grand, one sold yesterday on ebay, whole car, running for £400.
G60 would be fun, and slightly unusual in a Mk3 Cab, but harder to find and install and the running costs would be high due to reconditioning the charger.
How about a chipped Mk4 TDI engine ?