That heavily depends on your foil (especially the front wing area). SUP foil get out much earlier than kite foils etc. I would say 8 - 12 Km/h is a good estimate.
Depends on your weight, roughly 30 kg to get on the foil and 10 kg when foiling.
You can find an overview over all propulsion types here: Propulsion Type Overview - Wiki
Short version (my opinion):
Inrunner with gearbox
- works well (many do it)
- low drag due to small diameter
- no cooling issues
- low, predictable current draw -> cheaper ESC
- high complexity (seals, couplers, bearings, gearbox)
- high cost
- prone to failure (gearbox wear, seals, shaft/prop unscrewing etc etc)
Direct drive outrunner with water cooling
- works well too
- as simple as it can get, no seals, no gearbox, very few parts to buy and assemble
- very reliable, not much that can fail
- cheaper overall
- need to modify motor to prevent corrosion in the long run
- need to have an eye on cooling (not too much epoxy)
- need more expensive ESC that supports current limiting
As for the difference: Motors with high KV (-> high RPM) are more commonly available, but they are too fast for efficient props and have not enough torque, so we use a gearbox to reduce RPM and increase torque. Outrunners are available with very low KV and high torque so they can drive a prop directly.