First you'll want to find out the specifics on all of the hills you have to deal with; their length and their steepest slopes and the average of each one. You'll also need to know the worst-case headwinds you must deal with on those hills (and on the flat areas).
This is very important because along with the weight of the converted vehicle (which will probably be heavier than the original, by a lot if you need a big battery), and the speed you must maintain on the hills, it will let you calculate the minimum power required to do the job you need. THat lets you know the smallest motor and controller you could use, and gives a starting point for the minimum capabilities your battery will have to have.
Then, knowing that, plus the total range you need, will let you guesstimate the minimum amount of Wh you will need in your battery pack, so you can guess it's minimum required size.
Those will then let you figure a minimum budget you'll have to have available to buy the motor, controller, and battery, then you can guesstimate the rest of the stuff needed to complete the project.
You probably don't want to use a 400 pound motor, either--and if that motor is intended to run on regular single-phase or two-phase AC power, it's not likely going to operate from any typical available battery-powered controller you can get. If it's meant for 3-phase power, it might work with a 3-phase controller for the specific motor type (whcihever type it is).
Most forklift motors are very heavy (because forklifts have to be anyway, so no reason to make them light), and are usually brushed DC motors, commonly series-wound, usualy 24-48vdc. Controllers for these are easy to find, and you could even just use the one from the forklift if it will supply the power you need, if it is a throttle-controlled type (some just use a bunch of relays to switch power on or off). Plenty of forklift-motor conversions on the DIYElectricCar forums over the years, if you want some reference material.
There's no need to rewind your motors for high voltage. Just use a controller and battery pack that's matched with whatever the motor is wound for. You're going to have to design and build something that takes the output of the motor and runs it into your transmission anyway, or figure out how to directly run it to the drivetrain if you dump the transmission along with the ICE. When you do that, you just gear the coupling system appropriately to match the motor's output speed to that needed by the input of the transmission or drivetrain.
FWIW, there's lots of scrapped EVs out there nowadays in junkyards and even on Craigslist and the like, that you could get a complete EV drivetrain from to "drop in" to your car (possibly with some machined mountings and adapters for it that you'd have to design and make or have made for you). You may or may not be able to use the controller that comes with it, as it might depend on the other car's computer and sensor system to operate, and you may not be able to transplant all of that into your car. But there are various aftermarket controllers out there that can operate the EV drivetrain motors. You might end up having ot learn a fair bit about the controller(s) and motors and such in order to setup the controller to work with the specific motor, however.
YOu may even be able to adapt the battery from a scrapped EV to your car, at least the modules out of it if not the entire unit as a whole.