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...
What you sent me is extremely Finished, it's one of the nearest things to a coherent product I've ever received in the "stuff that plugs into old computers" category.
Now I've played with the card properly, I want to say this is great. It really feels like installing a "finished" product, with the Installer and driver and so forth. It worked plug and play for me in some rather demanding circumstances, and I like it a lot. Well done.
It could well be a specific one: around this time, cards and monitors were often sold as a bundle if they did anything unusual at all.
You'd have to look at what's actually being put out by the card etc to get some ideas.