It's Community Day. Provided you'll be catching for at least an hour I would evolve ALL of your good Charmanders as well as a shiny or two. Also, evolve any high level Charmanders you find. Once the event starts, you won't want to waste too much time IV checking Charmanders, and after the event non-Blast Burn Charizards won't be nearly as useful despite IVs.
Charmander to Evolve
I have 2 charmander, both 89 IV's, would those be good enough IV's to be viable for long term use (at least until heatran) or should i try looking for something better?
Answers
If you haven't caught a better one by 5min to the end (whatever time that is in your time zone), then those two end up being good enough to evolve.
But in the last 3 Community Days (I mostly skipped the Pikachu because (bleep) that little rat-battery), I've managed to catch at least 1 Pokémon with IV's of 89 or better and was at Level 30 or higher. So if you're planning on actually grinding during Community Day, you'll probably get at least 1 better than those two.
So grind for something better, but if the "R"NG gods don't favour you with a better one, those two will be your best to evolve.
Pretty much it exactly.
If you can't spend the 3 hours grinding for a higher IV Charmander, you've got two that are worth evolving. But if you can grind, don't evolve them until the end and you're sure you don't have a better one. (Even a lower-level one; by the end of the event you'll likely have enough candies to evolve a Level 1 up to a Level 40, if the last 3 Community Days are any indication.)