Kromlech at least used to have a lot of bits packages, and maybe still does.
This is 28mm Sci-Fi, so it may be that your troops' radios are just integrated into their helmets. Beam communicators, like lasers and masers, on the other hand, require somewhat larger hardware at any technological level.
You might be able to modify a figure's weapon into a beam communicator by putting a disk on or near the end of the barrel. The disk would be the receiver. My first thought was disk, but it doesn't have to be round. Any plausible shape would work. A nicely cut piece of stiff paper (like a business card) or foil (like the cover of a yogurt container) could be glued into place and painted as desired.
You can improvise almost anything to be a bag or carrying case for your medic or mechanic. I'd start with looking over old sprues for something suitable. Cut to the shape you like, glue into position, paint.
Current US Army medics carry a backpack with medical gear. If your troops follow that approach, minor modifications to the existing backpack might be all you need for our medic.
Larger tools like bolt cutters, sawzalls, and dynamic entry devices (25mm-scale Mjolnir would probably work) might require some shopping around.
paints another of 
