Which mon to power up

Personally I wouldn't bother with defenders. I can get 50 a day regardless of what I put in gyms.

Of your attackers I would only consider powering the Ho-oh or the Espeon if you're looking to be able to solo Machamp raids.

Absol is a much weaker Tyranitar and a steel moveset Scizor is going to be made obsolete when Metagross comes out and doesn't currently have a use in raids that another type couldn't fulfill, and powering one up just to deal with the occasional Gardevoir in gyms isn't worth it in my opinion.

I would not power up any of them, concentrate instead on hunting good attackers from the wild and hitting raids like Machamp, Tyranitar, Kyogre.

Ho-Oh would be the best of the attackers. If a tank for Machamp raids and a slow but potion efficient gym attacker is what you want, Ho-Oh is a good choice.
Espeon is not worth stardust from 20, it's much better to catch a wild eevee with 750+ CP and evolve that into Espeon.
Scizor isn't that useful and neither is Absol.

As for defenders, Blissey, Chansey, Snorlax are the only defenders worth powering up and only if you live an area where gyms change hands a lot. Steelix is arguably better when it's not powered up because it'll decay much slower and still be fairly tanky.