[Xracer]

I can see you rate the range of Kyosho cars and the MP9 certainly takes my fancy too if I were to try 1/8 off road again, I have just been looking at the S-Workz cars down here in Cornwall having dropped into their local circuit to see what is going on today!

To answer some of your questions though is easy whilst some others are purely down to budget and personal preference.

Lets start with the SG crankshaft, this came about many years back now for the 1/8 on-road circuit cars to enable the fitting of a true running clutch bell.
This was always a problem on the engines of that time that just had a threaded portion for fitting a prop nut as they were based on aircraft engines, so a threaded adaptor was required to fit a clutch and could never be set to run completely true, this caused vibration robbing performance and with the risk of the clutch adaptor coming loose, not good news!
The 'SG' style crankshaft has the usual threaded portion but is then extended with a plain diameter portion ground concentric with the main bearing diameter and it is on this portion that the clutch bell and drive gear rotates.

When it comes to motor choice then there are the 'budget' range of engines at around £100+ or the ranges of higher performance engines with more transfer and scavenging ports in the cylinder liner, their price range extends beyond £400 for some of these 'jewels of performance' but are only really needed by the 'serious' racers chasing glory!

Servos are a very personal choice but for steering and brakes, torque and holding power with reasonable speed are the obvious requirements and here again the price ranges from budget at £40 ish up to £100+. I would be looking at 10kg minimum with speed of .15sec or better.

Schumacher do a range of 1/8 buggy tyres in their 1/10 compounds and the same rules of application appear to apply, so an easy cross over here!

I hope some of this helps but it is difficult to be very specific on what are personal choices in most cases, as for the rest of the items you have already stated what is needed but don't forget the fuel!!!