You are probably pretty close. Of course it depends how much wing area you have and how you plan to fly it. You could probably even use a 120 although it might not have as much power as you would like. If you aren't too concerned with performance and want to go cheap(relatively speaking) you might consider getting a reasonably priced gas engine such as a Ryobi 31, US 35,41 etc. Then you will save even more on fuel.