Get something complete. Parts, even used ones, are relatively expensive, for no good reason I can see. Even if something doesn't work, it probably can be restored for a lot less than it would cost if you had to source one used or new.
Any car you find will have minor changes, compared to its original configuration, and as long as those changes are minor, that's OK. I'd avoid something that is extensively modified or has things on it that are obvious and just plain wrong. This quickly becomes a matter of taste, of course, but I'd still err on the conservative side.
As with all classic cars, you should think long and hard if you will have to depend on shops to do the maintenance and keep it running. You should be able to do at least basic maintenance and light repairs yourself. Otherwise, it not only gets expensive, but you'll spend a lot of time waiting for it to come out of the shop, and finding that the repair isn't right, having to take it back and so on. That is, if you can find a trustworthy shop in your area that can do it.
I'm currently restoring a TD; see the link below.