2.2 inch wheels are readily available from Kyosho, Tamiya, Academy, etc anything with a 12mm hex drive will fit.
It should handle a 19 turn no problem, we used to run 13-14 turns in them. The only problem is the idler gear which can wear rapidly, which is why the spur and idler gear is one of the few spares that aren't available any more.
Not much if anything breaks on Mids so you shouldn't worry too much about not having any spares. Wishbones are useful to have, but the later versions are much stronger than the earlier ones. Most of the spares are available to order from Kyosho, see here for the current list of available parts. The mid parts begin K.OT
http://www.tamiyaparts.co.uk/tamiyaparts/tpx/kyosho.htm
Bodies for both short and long wheelbase Mids are available from Kamtec.
If you want to stiffen up the gear diffs use Tamiya Anti-Wear grease, stays stuck to the gears.
Standard servos run fine on 7.2v, just be aware it supposedly shortens the life but I've never killed a standard servo directly connected to the battery.
Gears are 0.6 module and are readily available from RW Racing. Tamiya sells 0.6 module pinions as AV pinions and are also readily available.
Gear ratios as above, you don't need internal ratios as you can't change the spur gear, it only has a 4mm diameter hole in the middle.