Hello matey and welcome.
As good a place to start as any is here:
http://www.madabout-kitcars.com/foru...ead.php?t=3526
It's got links to a number of completed cars.
15" wires, yup - possible and I had them on my old Spyder, there is a link to the build and on-the-road thread on the above page. Just be prepared to alter the rack or apply garage-tool-No.1! (big ammer!)
"MGB Hive" should be your first port of call if you're after a 2nd hand set!
As far as Donors are concerned, Spitfire/Herald/Vitesse, I don't think there is a hard-n-fast reason to choose one over another. Personally I prefer the straight-6 in the Vitesse!
If you want to source a donor, and for _lots_ of help with bits, try "Spitfire Graveyard" as they normally have a whole tons of stuff lying around! e-bay tends to be a bit overpriced as that is where all the sofa surfers and dreamers go, but if you put in some legwork ringing around the old-skool suppliers you can normally get a much better deal!
You've already picked up on the fact that this is generally a nice bunch of people on these forums who are here to post, chat and blatantly steal each others ideas
Best of luck with whatever you do!