Firstly if you buy any kind of pre mixed fuel it is always going to be more expensive than if you buy the components and mix it yourself. Over in Oz mixing the fuel has cut the cost downby about 55%. I have a Tiger 60 and I fly it with a TT 91 FS. Doesn't need that much power but I had the engine and it needed a home. Overall is a good combination. Don't know how much your Gas engine is going to cost but it all seems like a bit of over kill to me. Carrying separate fuel cans, different pumps etc. If you are going to go with Gas build the appropriate size plane because the engine you are talking about soundsway to big for a Tiger 60.

I mix my own fuel and have for a long time, buy yourself a good measuring jug from a chemical store, buy the components separately and mix you own, you will save. This is an expensive hobby, trying to save a few bucks and cut corners or use inappropriate motors on aircraft is pointless and generally leads to disaster and a big bag full of balsa. The manufacturers put a rating on the plane to fit in with design and engineering parameters.