When looking for suitable miniatures in 28mm to play Geheimkrieg, one of the first stops must be West Wind Productions and their excellent Secrets of the Third Reich range.
The German Mech Grenadiers (above) are useful figures and like most of the range come with separate heads which allows you to replace the helmeted gas mask ones you get with the models. The German range also includes zombies and werewolves which are must haves for Abteilung 848 players.
The British Assault Troopers (above) are again useful figures, especially those with their Assault Brens, a shortened Bren gun which looks like the kind of hand weapon Blue Force troopers would sport. As with the Germans you can replace the heads and heads in para berets make a good alternative. The British line also includes a Boyes anti-mech pack, just what is needed to take out Mechanischritterkrieg, though I wouldn't fancy the recoil given the figure is standing firing!
The US infantry sport torso body armour and probably aren't ideal for the Dirty Dozen look of the 10th Special Service Brigade, however the Jet Rangers pack (below) is extremely useful and the figures modeled flying or on the ground.
(all photos copyright West Wind Productions)