It is hard to answer this question without knowing what planes you have in mind.

Personally, when someone asks me what plane and engine to go with when FIRST getting into gas engines, I recommend the AEROWORKS 29% Edge. It is a bit pricey, but is a really good flying plane in the UNDER 90" CATAGORY. This is a kit though and would have to be built.

With that being said, ANY of the name brand 50cc engines would be a good choice. The ZDZ 40, BME44 and the Taurus 2.6 would also do a decent job of flying this plane. For the little bit of price difference, I would go with a 50cc engine. Most of them get good gas "milage" so you can fly for 15 minutes on about 16 oz of fuel and have reserve.