Rhydon

First off, congratulations on that find! It's nice that it is close to lvl 30 already, so you won't have to spend nearly as much stardust/candy as you would have if it were a lower level!

If I were in your shoes, I'd save the 100% Rhyhorn for Rhyperior if/when Niantic ever releases Gen 4 Pokémon. Whether you decide to max it out now or later on, though, is up to you. But, I definitely would do it in a heartbeat!

Hold on to it but do not power up or evolve until Rhydon becomes meta-relevant again or Rhyperior is released.

It depends how often you see rhyhorn in your locale. In my area, rhyhorn are extremely common. If that is the case, I would not use any stardust on any rhydons at this time. Since you already have two excellent ones, I'd still hold off on evolving/powering up until Gen4.

You might find another excellent one. You can evolve that one. I recently evolved a Lvl35 98IV to 3058CP bypassing all my other Rhydons.

It's a 100%, are you seriously likely to catch a higher level 100%? And even if you do HEY, two 100%s!! At the end of the day you will eventually evolve/power it up. At the very least I'd evolve it and if it get's a good moveset you have another option and it would be cheap to get it to Lv30. If it doesn't get a good moveset I wouldn't spend TM's trying to FIX it because of the Gen 4 evolution, but it's never going to get better IV's.

I would just catch a level 35 Rhyhorn with a decent Iv and use that, save dust for Groudon.

I caught a 96% L31 too, and I'm still gonna save candies to evolve it.
If it gets a bad moveset after evolution, I'll just use it as a one-time use gym defender and keep it fainted till Gen 4.