Congrats on the car also nice body.
I have run the Super RS4 using 4.5T motor and 4600 NiMh cells at large 1/8th tracks some years ago what had been much fun. The problem is this car is very old and the drivetrain is not very good at least not for high power. Today the Lipo batteries are much more powerful than the old NiMh batteries so itīs even more load at the drivetrain with the same motor. I would definitely stay with 2 cell Lipo and chose motor depending on track size but I think itīs a good idea staying with something as 13.5 or 10.5T motor for normal tracks.
For parts list look at the HPI website maybe they still have the manual online for download. But itīs very difficult getting any spares. Some parts maybe could be used from other RS4 cars but I think drivetrain parts will be limited to Super Nitro. But usually this car just is wearing drivetrain parts and is strong beside this. You just should keep away from running with heavy nitro cars as I have seen also chassis breaking if getting crashed by them.
Ball diff is no good idea I have heard from fellow drivers the gear diff last longer than the ball diff. Same is for the CVD. Especially if you plan using fast motors as 4.5 or 5.5T the HPI CVD are not good as their joint are breaking. There was a CVD from GPM what was much more sturdy but the thread at the wheelaxle was a little too short so there was a good chance loosing the wheel nut. I used a GPM CVD for the front but had to reverse the locking nut so the plastic locking inserts was towards the wheel.
For the rear the original dogbone is fine for all motors but itīs a good idea using a special very thick grease at the junction between dogbone and wheelaxle. This is the most wearing part at the car. Very good grease for this was a black grease from HPI or something from Tamiya. Donīt know the name currently but itīs for high load applications.
As for other hopup parts I think itīs a good idea replacing the original plastic shocks with good shocks from Tamiya or HPI/Hot Bodies. This is what I used.
One of my favourite hopup was the oneway as the Super RS4 is understeeringering a lot with a frontdiff. This way you didnīt need that much steering throw also what improved the lifetime of the front driveshafts also. But I think itīs very difficult finding an oneway these days.
If you like running at fast tracks the Toyota GT1 body is the one with the most downforce and best handling. One of the best looking Super RS4 bodies also but the Lamborghini is very nice as well.
Anyway if you run the RS4 from kit or using option parts this is a car making lots of fun to drive.