[Ecky]

Yeah I understand you don't want to move the vehicle solely with electricity, I'd just like to throw out there that if the motor you're using is not bolted to the engine's crank, you can, in theory, move your car with it without needing the engine running. Like, let's say for instance you just need to move it up some ramps, you won't need to bother starting your car, you can just drag it up using the motors. Or, let's say you're at a drive-through, you can creep up to the window without the engine running. Saves gas, saves engine wear.

Anyway, the next step would be to decide just how much battery you want, and just how much power you want from the motors. Be aware that if you wanted to produce 200HP from the motors, it would mean your batteries would need to be able to safely produce 150,000 watts (750 * 200). If you only wanted 20HP, you only need batteries capable of safely producing 15,000 watts. Different batteries have different rates they can be safely charged or discharged, so you need to find the right battery for your application.

There are two major types of lithium batteries:

Lithium Ion batteries have very high energy density - which is to say, they weigh very little and are very small for the amount of power they hold. They are the 18650 cells you see used in flashlights / vapes and many other high powered devices. Tesla uses Lithium ion for their cars. Lithium ion is also cheaper than some other technologies. Its disadvantage is that the lifespan of these cells isn't quite as long as some other chemistries. I'm going to make up some numbers, you might need a new pack in, say, 15 years instead of 25 years if you used something else. Lithium ion can also be dangerous if mis-handled. If you short them out, they can catch on fire.

Lithium iron phosphate is the technology Nissan uses in their Leaf EV. These cells are heavier and bulkier than lithium ion, and tend to be in square pouches. They're safer than lithium ion, but more expensive. They will last more years before needing replacement - though honestly, either will probably last you plenty long, and battery prices are dropping fast.

Many older EVs used lead-acid batteries (like your car's starter battery) or Nickel-metal-hydride (like my Insight). These are extremely heavy for the amount of power they hold, but can be cheaper. With lithium coming down in price so much though, there's almost no point. The only case I can see for them is that lithium batteries don't perform as well in the cold as lead acid or NiMH.

The easiest way might be to find someone who is selling a Nissan Leaf battery pack, and ask if you can buy just 1/10 of it. Or, maybe you can find a pack from another hybrid like a Prius, Insight, Accord, Camry, C-Max, Fusion, etc etc. Or, maybe part of a pack from a wrecked Chevy Volt.

I'm not sure why you'd want to run a block heater from your battery if you're already bothering to plug the battery in (which is unnecessary, IMO), because you can just plug the block heater in too?