I would suggest getting a heatsink. Personally, a heatsink with good length fins are good enough, and a fan isn't really necessary. I could cook an egg on my Co-27 before i got an integy heatsink with fan. Well, the fan overheated and I don't use it anymore, but the heatsink itself works really well. But... it was very expensive.
I know the 3 racing heatsinks go for rather cheap on ebay and with other shops.
The Viper is a nicely sealed ESC, which I know is water proof, which probably means that it would easily stand up to dirt and debris. If I were you, I'd save up for the 11T or 8T limit if you are going to get Mtroniks unless you know you're gonna stay over 15T motors.
But yeah, go with a heatsink right now. It will help tremendously
