The wheel will turn fine on grass or hard surface. The spring just acts as a shock absorber.
As far as engines, I have not seen a lot of negative comments on the Magnum 52.
You might also take a look at:
Aviastar 53 available from http://www.sigmfg.com $69.99
SK 50 http://www.atsrcplanes.com/skengines.htm $59.77 - but it is currently out of stock