Seagull 61" Sparrowhawk to be Christmas Build

The idea of the wheel pants (on the full-size) was to REDUCE drag!
They do tend to trip models over, though, if you fly from a grass field.

The first rule of thumb for an electric model of this type is - - 100 Watts per pound weight.
So - you need to check what size prop the manufacturer recommended, then find a rage of motors that will turn this prop.
Having done that, you have to look further into the motor specifications, to find out what VOLTS it will take. (The kv number is the number of revs per volt that the motor will turn at). That will give you an idea of the number of cells in the lipo pack, and the motor's weight.
Find a lipo pack of the number of cells that will fit into the available space in the model, and find it's weight. The bigger the mah number, the better, as it will last longer per charge. Look also at the 'burst' power of the pack. That's not when it explodes, it is the short term max output the pack can deliver. (25c is average - or 25 times the capacity, so an imaginary 1c battery can deliver 25 amps for a short burst).
Now - weigh the model, with the servo's etc in it, and add the weight of the motor and battery pack.
Juggle the numbers until you find a motor that will turn the prop, the battery pack it needs that will fit, etc, and that will give you the 100 Watts per pound. (or more!)
That gives you a 'ball-park' start-point.
You will of course need a speed controller, but the extra weight of these is negligible, maybe an ounce for a large one.
As Garry put, you NEED a Wattmeter. You cannot see amps of electricity, so you HAVE to have one, to see what is going on. It doesn't fly in the model, it is just a piece of test equipment like a tacho.
Then you need a dedicated lipo charger. A nimh one will not do, any more than your 12v car charger!
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Safety is vital. An electric motor will NOT stall if you get anything in the way of the prop. So, if you are just fiddling around, testing and checking - TAKE THE PROP OFF. (obviously not if you are checking the Amps the motor is using, spinning the prop!)
NEVER charge lipo's in the house or car, or the model.
NEVER leave a lipo plugged in.
NEVER switch the transmitter off, if the lipo in the model is plugged in.
Don't rely on your car battery for recharging lipo's at the field. You might find, after a few charges, your car won't start because the battery is flat!
As this sounds like it is your first electric model, there is a lot to learn, and I strongly suggest you get on with learning before you connect anything up.