Thanks both of you, Miroslav you are an example of why this forum is such a greater resource than Gearslutz.
Its true that the M3500 I got in package deal with the tape machine may not even work, but I trust the seller. I met him in person and we had tea before we sis the deal.
Sadly, I left the PSU on purpose because I was under the impression it was easy (as far as number of suppliers) to have a newer one (with better specs) made.
I'm sure the seller would ship it to me if I cover cost. I'm thinking I might not want the big board after all however. The cost of cable to patch everything and the fact that I'm a solo singer/songwriter who builds songs up one track at a time make it impractical.
Because GRS has their "Seller is Away" thingy on on eBay makes me believe I will hear from them again eventually. And that I will get a PSU to test it. At which point I'd try selling the thing at o ly a bit of a loss. I'm looking into newer options -- mainly just for monitoring the 16 tracks from the deck and I'm open to hearing anyone's configs on here.
I'll make it simple...
If you want to monitor 16 tracks from the deck...you want/need a mixer.
Since you now say the original PSU was available to you (I honestly don't get why you didn't take it along with the mixer even if you still planned to have a better one built

)...you should contact the seller and get the original PSU.
Even if you plan to now sell the 3500, I can't believe you would pay $1k for a new PSU, and then sell the 3500 along with that PSU at a serious loss.
You will not get even $500 for the 3500 (that's the sad state of older project level analog mixers).
If you're going to try and sell the 3500, it will be less costly to just get the original PSU, and sell it like that...but I'm not sure why you just wouldn't then use the 3500 once you have the original PSU...?
I'm a solo singer/songwriter for the most part...and I replaced my 3500 with another, bigger console, because with my 2" 24-track (I used to have a 16-track before that) I find it so much easier to work with the console for setting up cue mixes during tracking, and also for mixdowns.
I'm not sure what other consoles you're thinking about...but no matter what you get, you will still need the cables to patch everything together with the console and the 16-track deck.
TBH...if you want to have a real simple, minimal setup, with hardly any cabling and patching to deal with, where you can do just one track at a time...
...then get rid of the console and the tape deck and external gear, and just got with a DAW rig.
I run both...it's a pretty involved hybrid analog/digital setup, and I love my tape deck and analog gear...but for people that want things simple, compact, inexpensive, and almost maintenance-free, I will always recommend a DAW rig to them. It's just a lot less hassle for a lot of people these days.