Nothing wrong with the TAFT Viperjet, its a well built model for a foamie, and the stock servos and retracts will work fine on grass with some simple mods to strengthen the mounting areas and brace certain spots.
Don't buy it with fan/motor/esc, waste of money, buy it as airframe with servos and retracts then pick out a decent fan that will give it the performance it deserves. Take a look at my VIPER PAGE for some info on both 6s and 8s custom setups. The 8s is the pick of the bunch as far as speed goes, listen to the sound wave as it goes past the camera, clouds and low sky dun matter, its the sound wave that tells you the speed.

There is a thread HERE in the Extreme forum with the mods listed, flew mine again yesterday testing another fan setup, fast but smooth landing on bumpy grass and the new retract bracing worked perfectly.

Here is video of our 8 cell setup running Jetfan90 & HET Combo:

http://www.youtube.com/watch?v=h8wMvoqVqBI