Alun. I should think the geared motor will not be drawing much more, if any, than it would driving an X50 prop (what MFA issue with their kits) approx 10-11A. A 25A speed controller would be ok..however it will need some ventilation..a bit of air across it..
It should go ok but an 850 but it is not a particularly "hot" motor. I can never remember what others reccomend, various 700`s on more than 12V and so on. Depends how fast you want to go.
In theory, the 850 puts out double and 800..and my Fantome, also at 48" goes quite nicely with two 800`s so in theory yours should go just as well.. Whether or not the increased efficiency of a larger prop would make any difference??
I would start with the 850 direct on to an X50 prop, mounted with screws so you can easily take the mount out, and see what you think..go from there. Use a 5mm shaft as well (if not already in place). there will be loads of room to try other things if not satisfied with the performance. As charles suggests, a 12V Nimh pack of as large a capacity as you can afford will be the best bet.
Ashley