Hi, The B4 slipper is a good upgrade and allows you to use modern associated spur gears and after market slipper pads.
The CVD it is a good idea in as much as you can use B4 3/16" wheels, but the drive shafts are too short. the fix it to use the B44.2 rear center axle part #9911
It is 1.25mm longer.
If you have enough 1/4" 2.2" wheels dont bother.
You can buy old style wheels in 2.2" (1/4" rear axle, 3/16 x 5/16" flanged bearing fronts)
over at JC racing.
http://www.jcracingproducts.co.uk/clearance/rc10
I think worlds car has front 3/16" x 3/8" bearings. bearings are cheaper than changing axles and all your wheel for correct offset.
Basically only upgrade is slipper the other are more to do with wheels.