On a side note, your idea doesn't actually add anything to mechanised build. Think of it. Right now if you want both fast mechanised unit (probably luchs) and base healing, you would go mechanised first, then you would build the luchs and go for the second truck with its upgrade. With your changes, you would have to go for mechanised, upgrade it to produce units, build luchs and then build second truck. That way you pay exactly the same, but you get the luchs later and you can't build units from battlegroup. The only advantage you get is the free repair upgrade, but that comes before any units you could repair, unless you go for multiple kubels. For most builds, single luchs and single kubel can be quickly repaired by sturmpios. Base repairs really becomes important only at medium tank level and up.
The advantage is you're not paying 200 MP 25 FU for the Battlegroup's unit production. You're right in that having to pay 100 MP 15 FU for the repair pios largely offsets that so you'd have to change the starting fuel to put all the rush times back where they were.
Another (probably better) way to do it would be to structure the trucks like this:
100 MP 15 FU to build a SWS Supply Halftrack.
Battlegroup Headquarters (0 MP 0 FU)
- Mobilize Battlegroup (200 MP 25 FU)
Unlocks the truck's units, the FRP upgrade and enables the truck's reinforcement ability.
- Field Medics (100 MP 15 FU)
Adds medics.
Mechanized Regiment Headquarters (0 MP 0 FU)
- Mobilize Mechanized Regiment (200 MP 50 FU)
Unlocks the truck's units.
- Repair Pioneers (100 MP 15 FU)
Adds repair pioneers.
Panzerfausts and the Schwerer Panzer Headquarters require a Mobilize upgrade to be purchased.
That'd work pretty much exactly as now except if you just want medics you pay 200 MP 30 FU instead of 400 MP 55 FU.