Would it work as a 2 to 3 month temporary psu or is it unpredictable? (The power supply has had about 100 hours of overall use max)
I'd be surprised if it powered it TBH, it's not a great brand & the 12v rails look weak with dual rail 13A + 14A (Note: you can't add them together to get the total amperage). You might get away with a modern, low powered card like a GTX 750, one of the models that doesn't require any additional PCI-E power connectors.
Bottom line, i wouldn't recommend using that PSU with a GTX 570.