Generally speaking for your drive system you are going to want a closed loop hydrostatic drive 1 for the left And one for the right they do make a tandem pump with a piggy-back auxiliary for the rest of the hydraulics on the machine. You could go with a pair of geroator wheel motors are used on exmark, Toro, Snapper, ETC, Zero turn mowers https://www.amazon.com/dp/B081NXSMGT...xpY2s9dHJ1ZQ==
Combined with a pair of these for pumps
https://www.griggslawnandtractor.net...variable-10cc/
plus you will still need a small gear pump for the lift hydraulics
I don't find the piggyback pump set
I have several of the wheel motors but the ones I have are much larger than what you are looking for and I have a quad pump set but it is 4 fixed displacement pumps connected together.
You also did not mention the engine you are planning on using I would recommend not smaller than a 21 to 24 HP gas or diesel
By using the above mentioned variable piston pumps you will only need some linkage connected to the pumps to make the skid steer go forward or reverse