Welcome to the site. You have a pretty well composed idea of what you want to do, thats very good!
I have some info for you:
Never never use starting batteries for an electric vehicle.
They can not handle many cycles of any decent DOD. They will simply die very very quickly. Yeah, they can put out crazy amperage, but they're just not meant to be drained much at all. You need deep cycle batteries.
Depth of discharge for lead acid batteries is usually kept to 50% to ensure maximum life. This means your 4.3 kWh is now 8.6 and your 9 kWh is now 18. This is a huge benefit of lithium as it can go to 70 or 80% DOD and still survive many more cycles than lead.
Most motors are rated at continuous output. Their peak power can be significantly higher. I had a series wound motor on my electric riding lawn mower that was rated around 1.2hp. I was pumping about 5hp through it while in use. It did get hot but it did its job for over a year like that. If I would have put some cooling on it (a blower fan or something) I think it would have lasted just fine. Unfortunately I didn't and on a very warm summer day it did overheat. I also don't think you want to mess with combining a few different motors. It'll just be a big headache. I'd suggest finding a decend sized motor that weighs a good amount.
I would definitely look up the open revolt controller here on ecomodder. It was designed by one of our members. If you're handy with a soldering iron it is insanely reasonably priced controller for $600. It will handle up to 144V and 500A.