There is one servo for thr/br.
Centre skids - I like the look of the ally ones and they are great protection, but, MOSSDOG (available through this forum) makes a plastic skid which just clips in and is dirt cheap. It looks a bit wierd, I'll admit, but it works.
YES to the air filter (Motor Saver or HPI High Performance)
YES to the fail safe.
YES to the Steering servo upgrade (Hitec 645mg)
YES to the throttle/brake linkage mod.
Everything else can wait IMO.
PS. sorry to bring it up again, but, What is the point of a slipper clutch if it doesn't save the gearbox?????? Yep it's your weakpoint.