Tamiya kits--both old and new versions--are really good starting points for those figures. You can modify them with techniques described in Shepherd Paine's How to Build Dioramas book--and/or just be creative!! Sand off details as desired, or, use a Dremel tool to modify clothing folds and details as needed. You can also easily modify clothing/uniforms with sculpted putty, masking tape or facial tissue (i.e. Kleenex).
Korean Conflict era US military uniforms were very similar to WWII US military uniforms--many times, just surplus WWII stuff. Your best choice is to do some research (watch the show a few more times!!, look on the internet or in books) for pics of uniforms similar to what you are wanting to model. The clothing then was primarily an OD green that faded based upon manufacturer dye process, laundering, wear 'n' tear, etc.
As said before, Revell made a few 1/35 scale M*A*S*H kits--one a Bell 47 and the SWAMP scene and I think another one with a 2 1/2 ton truck. I've never seen them in person, just on eBay...so can't comment on the quality of the kits. Actually...correction: I HAVE seen the helo built-up at a Model Contest and it looked great!
For the Dodge WC-54 Ambulance, Testors/Italeri, Bilek and (old) Peerless make that kit (actually the same kit, just reissued). I have 3, built one and halfway done with the second: an EXCELLENT kit (in my opinion) that can, with a small amount of work, be made into a superb representation of that ubiquitous vehicle!
Hope this helps! Keep us posted on your progess--looking forward to seeing what you do with this idea!!! Good luck!