This is a heavily racer-centric forum, so you're going to get racers' recommendations..
Whether or not you really need a $1400 full-on decked out race kit (add in decent servos and a radio and a couple sets of mounted tires to the above..) to bash around with friends I don't know...
OS engines are known to be really good, and user-friendly too.. Easy to break in, etc..
The Picco is overkill in a buggy, but hey, why not if you're not gonna' be doing organized racing?.. Just go +1 up on the clutch bell...
There's a guy here on rctech named janderson78 I think - he was selling Sworkz buggy kits really cheap a while back.. If he still is, and you want a top-flight buggy kit for half the price, look him up...