Hi Chris (and Dad),
1) Yes, all wood is fine, if you take the trouble to build as light as possible.
If I looked at the right boat, it's going to be about 80 cm long, right?
Take a look here for materials used: http://www.modelboatmayhem.co.uk/forum/index.php?topic=34033.0
This one comes rather close, just a bit larger:http://www.hobbyking.com/hobbyking/store/__16521__Princess_V_Hull_Boat_1000mm_Fiberglass_Hull_Only.html
At 'only' 80 cm length, you need to build very light, to be able to maintain a realistic waterline at rest, bigger would be better, as you'll have more buoyancy.
The under water hull will look something like this:http://www.r2hobbies.com/eng/proddetail.php?prod=rcbt0028_code
Notice the spray rails and strakes.
I have two of these hulls and each one weighs about 2650 grams, ready for the water, the topspeed with the stock motor (3650 1880KV on 4S lipo) is about 65 km/h (40+ mph), runtime 6-7 minutes full throttle.
Just to give you an indication of the weight/speed relation.
For a yacht I would not want it to be much heavier than 3-3,5 kg when 80 cm long.
2) If you want long runtimes and performance, a single motor would be the most efficient solution, as one big motor can turn a bigger prop, which in turn are more efficient than smaller ones.
Motorwise you have the choice between brushed and brushless (inrunner and outrunner).
In general the brushless motors are lighter and provide considerably more power with the same motor size.
Of the brushless motors, the outrunners provide the highest torque compared to similar sized inrunners.
There are steering outdrives available, like on the Fairline Targa 38, but these are limited on the power that can be applied, most likely the wrong scale and horribly expensive
3) The extensions are support for the swimplatform, so a flat transom is no problem.
Let us know if you have more questions