I'll try and answer some of your questions Rich...
1/ Standard size servos fit fine.
2/ I cant help with the ESC as mine has the Tamiya MFU sound unit which has a built in ESC, but cant see why a standard one would not work. you could always try contacting Hunter systems, he has ESC's and is only around the corner from you?
3/most cabs will fit your chassis, they attach via a pair of hinges at the front so the cab can tilt forward, As I said most of the chassis are the same with just the cabs being different. the bonneted King hauler or Aeromax may well bit different but I dont think your after that type anyway are you?
4/ yep you can leave the interior out or make your own, in fact a race interior would look really good, but I'm not sure if the Volvo still comes with smoked/blacked out windows. I have a feeling they did, but my Scania has clear windows.
Heres another one for you to think about, google steering set up on tamiya truck, there are a few mods you can do to give you far better steering. I did mine and you get a better turning circle and a lot less slop in the linkage.
Hope that helps
Ian