I'm all for DIY, but have you looked for used loaders for sale in your area? Even with free pipe or box tube you'll still have to buy a lot of stuff like cylinders, pins, bolts, hoses, make or buy a bucket, etc which all add up but they do get spread over a longer period of time so it can seem cheaper.
As far as dynamic fluid requirements, if all 4 cylinders were 24" long with 1.25" rods, the volume difference between fully retracted and fully extended is only 0.16 gallons so your 3.5 gallon reservoir is plenty. If your tractor doesn't already have an oil cooler then it may be worthwhile to add oil capacity to compensate, but I'd want to monitor the temps first before spending the extra time and money.