I've been designing machines with hydrostatic drives for a major OEM for many years so have quite a bit of experience with them. I will caution you that diving into such a project as you describe without a very good understanding of everything involved could easily end in less-than-desirable performance or (worst case) a very expensive learning experience. The components of a hydrostatic system are by nature quite expensive and in many ways are rather unforgiving of errors. I'll give you some hints here but more as an overview, not a step-by-step guide on everything you need to consider.
The first question to ask yourself when sizing your components is how much power do you want to deliver. Don't focus on torque - power is what really matters. After determining your desired operating pressure (typically in the 2500 psi range for a decent-sized hydrostat) you can then calculate the necessary flow rate to deliver the power you're needing. Once you know the flow rate you can then select a pump displacement that can deliver that flow rate at the speed that you'll be running it at. (Note that for hydraulic drives that flow, pressure, and power have the same relationship as rpm, torque, and power do when talking about mechanical power.) In the same way you use the flow rate to select a motor that will deliver the desired rotational speed at the pump's maximum displacement. The motor displacement can also be used calculate the maximum torque that you'll be able to deliver into the gearbox at the pump's relief pressure. Note that this peak torque will be about double the normal operating condition since you want the operating pressure to be well below the relief pressure to give you plenty of reserve capacity.
A closed-circuit hydrostatic drive also requires the use of a charge pump to keep the loop full of oil. Many hydrostatic pumps have a built-in charge pump which obviously simplifies the installation so be sure to verify this before buying a hydrostatic pump. It is critical to have a filter between the charge pump and hydrostat, something in the 10 micron range that is capable of 500 psi. The way this is plumbed into the system will be determined by the porting of your particular hydrostat. You'll also want a return filter that the case drains from the pump and motor are plumbed to.
Depending on the particular system and how hard you're working it it may be required to have a loop flush valve in the system to bleed extra oil from the loop for cooling. Some motors have a built-in loop flush valve that dumps oil from the low pressure side of the loop into the case drain. Since you won't know if your system will need added loop flush I wouldn't worry about this at the start - you can always plumb in a loop flush valve later if required.
And speaking of cooling you will almost certainly need an oil cooler to send the drain lines back through on their way to the filter and reservoir. Garden tractors and lawnmowers get by with just fans blowing on the hydrostat but for something like what you're looking at a cooler would be necessary. Plan to include a cold-oil bypass valve to prevent excessively high case pressures when the oil is cold and unable to pass easily through the cooler.
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