Hi Barry,
I'm not sure there are very many "chequebook" restos on DDK. Seems most of us like tinkering about, stripping down, refurbing and re-assembling what we can. It's part of the learning process, very satisfying, and often nothing to lose. For example I spent hours this week persuading the original oil level sender to work again. Had I failed I would have needed to buy a new one - nothing (other than my time) lost by trying.
And of course you are right - much better (for all reasons) to keep an original part if possible.
Problem is, if a part is beyond repair or missing, and a visit to the OPC is unavoidable, it's going to cost. Same with used and repro parts - my recent foray into Alfas has reminded my how silly some Porsche parts prices are!
And when we run out of skill and need a professional, many labour rates have also got quite high. Not yours, obviously

.
I think that, if aiming for a really high standard of resto, the list of areas where professional is best is fairly long. Metalwork, paint, engine (unless you are really skilled, do you want to risk your matching numbers case?), MFI.
Agree with you re strip down, refurb of minor components, most re-assembly, most interior refit and clean-up. Electrics also simple on these cars. I agree that lots of money can be saved by tackling that stuff hands-on.
Cheers, Richard.