Trying to save money has cost a lot of guys their airframes. If you will get the appropriate engine for the plane you are flying now, you will have the best chance of flying it successfully. When it gets time for a second plane, start from scratch there too. Buying an engine that's too big for the plane you are flying now, especially as a beginner, will just increase your chances of crashing. Buying radio gear with thought for the future is a good thing, but putting a too heavy engine on hoping to use it on your second plane is just not good building. A light weight 2 stroke engine is tailor made for a plane like yours and will fly it better than anything else.
Good luck.