Should I power up this Arcanine with these IVs?

I would not power it up. As long as you're a decent dodger you should be able to beat any pokemon in any gym as long as it is weak to fire types.

the question is, how often do you find good growlithes? If it's easy to find it in your area, just leave this big dog as it is, but if it's rare and you'll only attack with it, keep it and power him up cuz he'll be a attacking monster anyway. Think to yourself "what are my chances to find a better one?"

Nope. Just use him the way he are (dodging) and wait for a better IV (90+) to build your definitive dog. His attack will be perfect, but with 0 def he will receive much more damage so you will spend lots of hp/potions every battle. The big secret of this game is that to be patient while you use correctly what you have in hands.

I would power up.
What chances you have to evolve another one and get best move set?

I would power it up. Admittedly growlithes are rare for me, but even if they were average I think it's still worth powering. Unless you're able to evolve an Arcanine every 2 weeks or less, I think it's worth powering up.

FF/FB is the best moveset for attacking. You obviously want the fire type STAB on Arcanine. FF/FF would be only slightly worse, but still worse.

So, you need to find a 15/X/Y Growlithe with X+Y>25 to really make a difference over this one. It also needs to be at least level 20 to even be worth considering, probably at least 23. Then you need to spend 50 more candies to evolve it. Then you need to hope it gets FF/FB or FF/FF if you're willing to give that up for about 20% more IV.

So if you're getting one every two weeks, maybe you'll get a good enough Growlithe once per month and you can evolve a new and improved Arcanine in 2-3 months with the right moves. Anything longer and you're looking at like 6 months or more to improve on this guy - do you really think it's worth waiting that long?

This one is good enough to power up. Go for it. Even if you can get a lot of Growlithe, why not power two Arcanine then? He's a beast anyways - worth the stardust. He'll outrank Lapras in gyms if you don't want to attack with him anymore.

You've got a good moveset - I've managed three Bite/Bulldoze in a row...

Wellll that is 66% arcanine with perfect moveset.
It's going to take a very long time for you to find a better one - and all that time your current arcanine will/would be better.
I'd say go for it, people seem stare at IV as if they were the most important thing while they are the least important thing IMO

I wouldn't, I only power up defensive pokemon, they are the ones where CP is really relevant, having 2k+ CP is more than enough to attack into pretty much any pokemon you'd want to pair an Arkanine against so if you want him for attacking, you should just use it as it is and not waste your stardust.

Man, I wish trading was a thing. I have so many good arcanine's and about 400 growlithe candies to spare. Haha They're so common in my area.

After dragonites, arcanines are my next best gym attackers. Even though ff/fb has the highest dps in the spreadsheet, in real world test, I actually prefer bite/fb combo. Bite is extremely fast ( easier for dodging ) and charges the fb much faster. And in the event if u need to use your doggie against dragonites, bite is better as it is neutral dmg to them.

Here's a real world test on the 3 movesets - https://www.youtube.com/watch?v=IERtm8-UObI