The O.S and the Evolution are both great engines, and either would be a fine choice. I have both and have not had any troubles from either. I have also had the Sig LT-40, and it is a very good choice. The Plane is big, easy to see, flys great, and looks great.

Building is something you do because you enjoy doing it. I enjoy building alot of my Planes, but you cannot build a Plane for as cheap as what you will find it in the ARF version. The Sig LT-40 in the Kit version is a pretty good kit though I must say, and comes with a generous amount of hardware. It is fairly simple to build, and if you are thinking that maybe you would like to try your hand at building, it would be a good one to start with.

Covering does take a little bit of a knack, but I am sure you would be able to find someone who could offer you some help or tips. The Radio you have chosen will do you a good service for quite sometime until you get into advanced airplanes a couple years down the road. Sounds like you are off to a really great start, and have made some very good decisions so far. Good Luck, and welcome to the hobby!