You should check your numbers.
Full tech Ostheer into Pz4 is only 275 fuel I don't know how you find 285 for a rush, USF 1 tier tech full upgrade is already 305 fuel.
The only way for USF to reach that 285 fuel you mention is to build only an ambulance with 1 full tier. Good luck fighting Pzgren without BARs.
My numbers are just fine.
Ambulance is 10 fuel, as is the T1 Ostheer cost so it's likely they cancel eachother out. Even if they don't, the map control the ambulance gives with forward healing makes up for the fuel cost.
Grenade and rack are optional. I usually got weapon rack before the M4 and I got a lot of early map control which made up for the cost.