I'm sure a faction with the heaviest stock armour options including 4 units with over 200 armour combining repair dudes on mech, fast repair sturms and a 3rd multipurpose unit wouldn't present any balance issues. A pair of mech trucks already grants incredible bonus to repairing armour so why keep that limited to 2 okw players and not make a single enough!
It wouldn't be for free. If Soviets can buy 170 engineer, USF has crews and rear echelons, ukf has 5 men sapper squads that can be enhanced through anvil I don't see why okw couldn't play risk reward with a costly upgrade on a fragile truck. It would give them cool risk/reward mechanics, and added one more reason to build it. Sturmpios are expensive. If you don't lose them, everything is like you say but if you do lose them (happens a lot as they are constantly repairing sth lategame) you are in deep trouble. Repaying 320 manpower just to buy a Sturm that you will use only to repair is just expensive. I also don't think that what I suggest would be extremely popular, just another possibility to explore for some players. Unlikely to change balance, it would probably add some more colour to the faction, that's all.