How handy are you with a welder?
The youngest tdi will be what, 15 years old now? In this country that means it's very unlikely you'll find one that isn't rotten. If you do go for one, take someone with you to view who knows the usual rusty bits (sills, boot floor, inner wings, anything not made of plastic really).
As far as mechanicals go, if you know your way around your 90 it's going to be pretty similar.