It sounds like your on a budget. So here's the scoup ,4 stroke ,25% nitro and larger props are all double the money. ST 51 would be your best buy, Ringed engines last longer ,10% nitro is perfect and less harmful on the engines. 12x4 K prop is strong and long lasting in grass and asphault nicks. I get a big kick out of all these guys saying a 4 stroke will swing a much bigger prop ,but it is almost always 1/3 larger displacement. I bought a 2 week old Harrier 3D 46 that had a 82 saito on it. I put a ST51 in its place and the ex owner couldn't believe the same performance. APC 12.25x3.75 is also a great choice.