Great to hear you have everything important backed up!
Yes…a replacement motherboard for an Apple computer is expensive. And many times…it's not worth it compared to the value of the computer. I'm in fact right now…typing on a 2011 13" MacBook Pro…and there's no way I would spend $600 on it to have a motherboard replaced. Why?…a 13" 2011 MBP in 100% working condition isn't worth $600 bucks. Or at best…is only worth $600.
And on top of this. Why wait a month (30 days) for this place to repair it…when you can take it to an Apple Store…and they would probably have it done in less than a week.
Anyway. You said that you're mostly having trouble with the number keys on this computer. If you're also having trouble turning it on…then there must be trouble with the power key as well.
I know that you have concerns about this computer not starting back up at some point. But if it's going to fail at some point…then it's going to fail. One thing I would try is doing an SMC Reset:
https://support.apple.com/en-us/HT201295
I would say do this…and see if it helps.
- Nick
p.s. I should also ask.
- Why did the computer need the first keyboard replaced?
- How long has it been since that repair?
- Did the computer work fine after that repair?