Gear Speed 400 brushed versus brushless

Assuming that you get a brushless with the right Kv rating you should be able to turn the prop a bit higher speed at the same power as the current brushed motor. Geared brushless motors seem to really shine when you run them _fast_, which would indicate using a higher voltage (or lower Kv motor) and a smaller prop or more reduction in the gear box.
It all depends on what you want. If you want to go straight up forever with that poor little Cessna then you probably need to gear down more and put a really fast motor & powerful in there. If you just want sustained inverted flight then the right motor with that gear/prop combination may be just the ticket.
If your LHS is really good you could just ask there.

Yes, provided it physically fits, the brushless is an inrunner and will work at an appropriate RPM when geared, and you take account of the likely different motor characteristics when selecting the pack/prop.