Sorry, but as I read it, your 'buddy' sold you a 2006 mini, which is, for all intents and purposes, a turkey at this late date and not worth investing further upgraded components into, setting aside the question of whether you can put upgraded components into it.
As much as I understand and sympathize with all the mini users out there who want to extract as much value as possible out of old minis that date from 2006-2012+, my advice is to bite the bullet and just start anew with a 2018 mini. Yeah, it's pricey and Apple overcharges on SSDs and all the rest. But once Apple finally (finally!) came out with the new 2018 minis, I was more than ready to cut loose from my old plodding 2010 mini. I'm not sure what I will do with the old mini, but I don't plan to saddle a 'buddy' of mine with it. If I were you, I'd ask for my money back. YMMV, of course.