Buying a used computer from an unknown seller is always a gamble and you won't know how that machine was treated or did it have some major problems which the seller is forgetting to tell you about. Additionally, unless they follow all of the steps here and can prove it, I wouldn't touch it:
https://support.apple.com/en-us/HT201065
If your mid 2012 has the original slow 5400 rpm hard drive, I would consider swapping that for an SSD. However, that would mean more money spent which you probably do not want to do. And, as for the "outdating", that is the nature of electronics. They come out with a newer, faster, better, more features model all the time; then the software will be optimized to run on that and won't be compatible with the older hardware. That is what they all do - that goes for TVs, refrigerators, etc. as well. They wouldn't sell anything if it lasted a long time.
FWIW, I bought a mid 2012 in 2017 as a refurb from Apple with Applecare to 2020. I swapped the slow hard drive for an SSD and added more RAM; it is working nicely and covered until April 2020.