If you haven't made your mind up yet I would recommend just Getting Arma 2 CO (original and OA together) . Not that IF44 is bad, it is very outstanding, but I don't foresee Iron Front 44 getting any future support or any more DLC's...I don't know for sure but I think we've seen the end of support for Iron Front.
Regarding your question about what units are available, their is a class-list posted on official IF forums but those forums are blacked out (part of the reason I suspect they may be packing it in). Germans have Tiger I, King Tiger, Panzer IV, Panzer V and Stug 3 as far as tanks go. T34's, SU-2 and IS-2 for Russians. I think Americans just have 1 Sherman and their is no armored cars (APC's yes) which really annoys me personally