It sounds like you have a pretty good base already.
Since you asked for some suggestions, though, here's what I would recommend:
(1) Heavy Kommandotrupp. These guys have really cool abilities (particular Stimulant Kit) which give you some new tactical options.
(2) Lothar. Since you already have the Observers, you might as well get an artillery unit so you can use them!
(3) Axis Hero Pack. Stefan is a good basic hero that can add a flamethrower, 4 more hit points, and (most importantly) Assault to any of your Type 2 infantry. Grenadier X could be a great addition to your zombie squad, as giving them Assault makes them even more of a threat.
(4) If you want some more anti-tank punch, the Laser-Jagdgrenadiere would be a good choice. The range of 6 on their anti-tank laser is quite nice.
(5) It's rarely a bad idea to buy more Recon Grenadiers. They are very flexible, since they can take out infantry or vehicles, and their point cost is ridiculously low for what you get.
(1) Corps Officers. As with the Axis Heavy Kommandotrupp, the new abilities these guys have really changes the game.
(2) Steel Rain. I think that this is probably the best medium walker the Allies have, and maybe the best in the game. Rolling 12 dice against other medium walkers is just fantastic. And the Corps Officers will let you reload the rockets so you can do it twice per game! It will also give a use for that Observer unit you already have.
(3) Tank Busters. These guys are a fantastic anti-armor unit that are pretty effective againt opposing Type 3 infantry, too.
(4) Mickey. Don't expect it to do much against enemy walkers, but it can rip opposing infantry to shreds in short order. It has the Fast skill, so it can move quickly around the battlefield and it also has Self-Repair which makes it a bit more survivable than most other walkers in its class.
(5) Legio Patria Nostra. If you want to clear enemy infantry out of a building, these guys can do it like nobody else.