Is 37 Res good for ploys?

Yes.

Depends on who you are trying to ploy... but yes, I'd say the threshold is around 35~ to be above average res, though most t20 armours don't hit 30 at the start of the phase

Yes, it’s fine. My policy is 35 res or higher is fine for ploying.

Have you tried it?

If not, go to the list of heroes on this site, sort them by res, and see who has 37+ res. 34+ if you assume they use fury, lower the threshold a bit more if you expect to be outmerged. You'll see most of the targets you can't hit. Then you can decide whether 37 will do it for you.

All heroes have at most 36 (neutral) res. So it should ploy all non-merged, neutral, and non-buffed units.

Yes, and that's without buffs remember that lol

Either way, 37 Res is more than enough, with smart plays, any unit who you need to get Ployed will be vulnerable to it :)