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Battery size

I suggest looking for typical energy consumption rates, which are typically expressed in kilowatt-hours per kilometre or per mile. When you have a plausible value, multiply that by range to get the required usable energy capacity... and since you can't use the entire nominal capacity of the battery, you will need it a little larger.

Regenerative braking reduces the total energy requirement, but all production EVs do this so it is already accounted for in their energy-per-distance performance.

People can give you a number, but it will be based on their requirements and their understanding of your project... not yours.
 

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Motor sizing

... what size of motor ... would be required to give a top speed of 60MPH ...
The details are dependent of the power-versus-speed characteristic of the motor and the available gear ratios, but if a motor is just able to keep the vehicle moving at 60 mph on flat ground, it take essentially forever to get there and will not be able to maintain that speed while climbing any incline or facing any headwind.

I suggest defining some performance scenarios, determining what motor power is needed to handle them, and then looking for a motor to handle all of those situations, such as:
  • maintaining a target speed up a moderate incline
  • maintaining a lower target speed up a steep incline
  • accelerating to a target speed in a suitable period of time
These will all depend significantly on the vehicle mass. You have a target of 1600 kg, but how was that chosen? Since the battery mass is significant, if you independently choose the vehicle mass and the battery capacity, you have no control of mass available for the rest of the vehicle and cargo.
 

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You might want to take a step back and determine what the real requirements are. If you have assumed that regenerative braking would be best, that's a premature design decision; however, if learning about regenerative braking is a purpose of the project, then including it is a valid requirement.

The other decision that I noticed is the use of the original transmission; there might be good reasons for that, or it might be unnecessarily constraining the design.

There's also the mass of 1600 kg. I had read that as a target, but I now realize that I probably misread it - it is likely the curb weight of the vehicle before conversion. After conversion it will likely be heavier, so be careful with assumptions about the post-conversion mass.
 
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