I think I'm got most of the Medic figured out.

His outfit is easy. I'll tuck some dark slacks into black boots, and put on a white shirt and light-blue tie (all of which I already have).

I've got a lab-coat that my colleagues gave me as a joke when they ordered too many for the nursing instructors, and couldn't return them (the dean dared me to wear it to class for a week, and I did. My prize was the coat). Unfortunately, it's a women's lab-coat (it buttons from the wrong side) and it's couple sizes too big, but I think it's okay if the coat looks a little strange, and if it looks too bad, I could have Betsy modify it a little for me (if I ask nicely). I can paint the blue medic-cross on the shoulder easily.

The back-pack shouldn't be too tough. I could find the right size cardboard box, and use some poster-board, and a 2-liter bottle to make the doo-hickys. then paint the whole thing and put some shoulder straps on it.

The weapon will be the tough part: The medi-gun looks like the business end of a fire-hose, and I don't know where I would find one of those. I might be able to find a nerf gun that looks like the syringe-gun, and give it a paint-job, but I don't think that would look very good. The medic uses a 'field bone-saw' style that has been obsolete for decades (proly because they don't need to do amputations in the field very often anymore), and the only ones I could find for sale were really expensive antiques.

Out of the three weapons, the bone-saw would be easiest to make a mock-up out of card-board or a thin piece of wood. Some silver and black paint could make it look okay at a distance.

I've also seen the medic lugging a novelty-sized syringe in some of the pictures, like the promotional group shot. I could easily (and cheaply) pick up a prop syringe, but that would be a last resort, since it feels kind-of like skipping out on part of the costume.