From what I have been reading up on over the past few months, a whole range of factors can contribute towards the buggies handling/performance.
The differential oil can play a factor, shock position, springs, tires, ride height, camber, toe in, etc, etc, the list goes on!
I'm no expert on this by any stretch of the imagination but these might help though, have really helped me out, a very good read too!
Hudy Off-Road Setup Guide (.pdf file)-
http://www.google.com/url?sa=t&sourc...g6X0pb_XPPKy0Q
http://www.maxbashing.com/content.ph...Monster-Trucks
As for servo, for that price you should be able to pick up a decent second hand Savox off of either eBay or one of the many forums 'For Sale' sections.
Hope this helps!
Have just seen as well, that Reg does actually have a Savox servo for sale as well,
http://www.oople.com/forums/showthread.php?t=106352, good price too!