You might want to look at some of the Takky resources out there, they have a certain amount about wiring up these logic boards to things they're not meant to go in
I don't know, I'm just going from the sad mac on ifixit. But if it's not doing that any more, who knows.
I'm going to disagree very slightly with @Phipli here; I think a Plus is a very good first real repair project, because there's enough stakes to keep it interesting, but the computers are...
That seems a reasonable price to me - Pluses are very fixable, and if you're willing to take it on as a project, sounds reasonable.
If it is just the ROM, that's relatively easy.
As far as big component suppliers go, I'd look at Farnell and RS before mouser and digi-key; the latter are quite american-biased, the former two I believe have distribution infrastructure up in Norway, which probably means you'll get the bits faster and they have to go on fewer pointless air...