If the lowest voltage you switch is 7V, you probably need a logic level one. The non-logic level FETs usually want 10V to be fully open.
Do check the maximum rating of the gate-source voltage. E.g the IRL540 can handle no more than 10V, the IRLZ44N is limited to 16V. Your 18V would destroy both. A zener can limit this voltage. The higher the voltage at the gate, the lower the on resistance.
Do also check the on resistance. The IRLZ44N is a fairly decent 22 mOhm at 5V on the gate, but at 12A that's over 3W of heat dissipation, that needs a good heat sink! An easy way to reduce this is to use two MOSFETs in parallel. That cuts the total heat dissipation in half, and per device to 1/4, some 800 mW, so now just a little heat sinking is needed.
That's the "max" number as given in the data sheet, and what you should be calculating with. Basically a worst case. Then the heat dissipation at 12A is 1.7W. No serious problem. There are small clip-on heat sinks available for the TO251A package, which I assume you will want to use. For the DPAK consider a stick-on heat sink on the back of the PCB, with a good number of thermal vias. That'll also do the job.
This 1.7W is really a worst case scenario for you: a part with on resistance at the top of the tolerance, and an exceptionally low gate voltage. Most of the time your gate voltage will be significantly higher than 4.5V, as your battery starts at 7V. At 10V the on resistance is about half the value at 4.5V, and of course that means less heat. For this one I don't see the need to double up. Just add a small heat sink and make sure there's some ventilation to get rid of the heat. In reality you probably have less than 1W dissipated there.
The part can handle 46A so that should be OK even with motors stalled for a moment. Of course in case you have 30-40A flowing you need to switch off power soon as otherwise it will go up in smoke
Delta_G:
I have this huge transistor with a big honking heat sink on it. I know it can take the current: FCA47N60F - 47A - 600V. It may be way overkill. Even so I prefer overkill to buying something else.
It's certainly not overkill, its fairly inappropriate. 600V MOSFETs have very high on-resistances compared to lower voltages - this one is 62 milliohms on-resistance which means about 10W dissipation for 12A, requiring
a large heatsink
An equivalent modern high-performance 30V device might be 1 or 2 milliohms, and not even require a
heatsink.
Even a pretty standard older 30V device would be about 10 to 15 milliohms dissipating a couple of watts and
get away with a small heatsink.
