side mounting an engine is fine.

Just follow the instructions included with the kit they sometimes show different mounting methods for the engine.\


as far as the prop goes I've used 10-6, 11-5, 11-4 get one of each and see which combination suits your model and flying style.

By changing prop size and or pitch you can either get more speed or more pulling power.

Bottom line is to try different props and prop manufactures to find the prop that suits you best