I should focus on having high current, rather than voltage?

Generally you tackle battery building in this order:

1. How fast do you need to go? This sets the voltage of the pack. As

@brian_ pointed out, Motor controllers typically regulate the pack voltage to a lower voltage, so the pack needs to be designed to turn the motor at the maximum speed you need it to go.

2. How far do you need to go? Your range is computed by taking the energy in the pack (in kWh) an multiplying it by the efficiency of the vehicle (in miles/kWh). So for example if you pack has 10 kWh of energy and you have an efficiency of 3.5 miles/kWh then you have a range on 35 miles for that pack. You can improve the range by making the battery bigger, improving the efficiency, or a combination of both.

3. How fast do you need to accelerate? This is the torque which is motivated by the amount of current the battery can deliver to the motor. Generally making the battery bigger by paralleling cells facilitates adding more current to the mix, improving the acceleration.

You've only answered one of these three questions so far by indicating a range of 50 miles or less. You still need to set a top speed. Let's say 60 MPH/100 kph. For the sake of argument doing that computation with the speed that the motor turns, the final gear ratio, and the size of the tires computes out to 96V to generate that top speed (I'll offer to compute the actual numbers given the specific motor, final gear ration, and the actual size of the tires). To get 96V with your 3.6V 42 AH cells you'd need 27 of them. Cost is $700 at $25/cell. Total capacity is 3.6V*24*42 Ah = 4.082 kWh.

You can start to see the numbers now in the example. With 4 kWh such a pack will get you 12-15 miles of range. The lowest current limit of the motor, controller, and pack would determine the maximum current that can be delivered.

Each $700 investment into the batteries adds 12-15 miles more range and improves the batteries ability to generate more current.

But no one but you knows what parameters actually work for you. Answer the three (or at least the first 2) of the questions here along with your maximum budget, and then maybe we can figure out a design that can meet those parameters.

ga2500ev