I'll try to answer your questions:
1) I didn't find the need to fill the Lisa Figure with plumbers foam. However, I did need to glue her head on better. It also appears to me that board shark builders made the "dude" figure static (not sure how this is done. maybe just glue him in place).
2) I an not a motor expert. Motor theory can be quite complicated. What I do know is that 19T or xxT stands for the number of wire winding turns around the motor armature. I believe the higher the number of turns usually the more torque and less RPM. A 19T motor is typically a "Brushed" motor and is what a lot of RC cars builders use. I guess because it is a middle of the road motor selection (good trade off between power and speed). Most people speak of motors in terms of Turns and KV. KV can be used to determine RPM. RPM = KV * volts. So my RPM = 3300KV * 11.1.
As for sensor versus sensor less, I don't know much about this but it has to do with the motor and the ESC knowing the exact position of the armature at all times. Depending on where the armature of the motor is position at rest, depends on how much current is needed to start the motor turning. I believe these setups require that the ESC be a sensored ESC. I believe a sensor less motor will work with virtually any ESC.
My motor is sensor less and it cost less. What I have notice with mine is that sometimes It takes a second or two to get the motor turning I guess depending on where the armature has stopped. You may need to go sensor less if you want to use the Nicad packs (you will really need to research this more - try Google searching "sensor versus sensor less motors" etc.).
The water jacket is a jacket that slips over your motor which water runs through to keep it cool. Your Surfer comes with a water coil so you will not need a jacket.
3) 40+ amp ESC should work. BEC stands for Battery Elimination Circuit. What this does is it removes the need to have an extra 4 volt battery pack to power your radio receiver. instead it uses the main motor battery for both powering the receiver as well as your motor. This will save you space and headache not having to worry about two battery systems.
4) The Radio that comes with the surfer is junk. It is not proportional meaning when you steer you get full right or full left only and when you throttle, you get full throttle or stop. You will need to replace this with RC quality proportional radio equipment. These can be purchased for about $50.00 and they come with pistol grip Transmitter, receiver and usually two full size servos.
5) ? -
Your Surfer comes with a cheap charger. This is a trickle charger. Should work but it will take several hours to charge. Quick chargers cost more but charge packs in about 1 hour. I used Nicads for a long time flying airplanes but bit the bullet two years ago to invest in Lipos.
6) A new 2 channel radio will typically come with 1-2 full size servos. I like Futaba radios. It is reasonably priced and the quality is outstanding.
I hope that if any of this is information is incorrect, others will chime in to correct me.