You would have to save after the end of each game in a different slot then you could reload and repurchase but then you might get a different battle.
Sometimes in campaigns the better part of valour is learning when to retreat in the face of overwhelming odds and save your core assets.
You can hurt the Russian AI in this one but you are so short of infantry it would be next to impossible to hold for long. All those MG teams are nice for holding back waves of infantry but the AI dosen't always do what you expect it to......does it ?
The 88's are great but you need lighter AT guns to deal with the Light tanks so you can save the 88's for the T-34's and KV's.
( and no....... there is no "right" force to buy to solve every problem that game may throw at you but experience is a great teacher )
Don