(Apologies if some of this is more basic information than necessary, but hopefully you and others may find it helpful)
For most, if not all, "bad guy" costumes, the 501st Legion is the place to start - where things get complicated is that all of the costumes are split up into various "detachments" and generally the more obscure the character/trooper, the more difficult it is to find "off the shelf" costumes or even components.
So for example, if you want a standard OT stormtrooper you can have your pick of all the specific flavors (Sandtrooper, ANH Stunt, etc) and choose armor and accessories from a variety of sources. But for your preferred option of a medic, there won't be as many resources readily available so be prepared to have to put in some extra work if you have your heart set on a specific costume from a particular film/series/comic/etc.
The good news is that the 501st is a large and pretty well-organized group, so you can generally find good information on most costumes and other builders, you just have to be prepared to potentially search the forums for multiple detachments to find all the information and/or help you may need.
There are two "medic" options in the Imperial group that I could find on a quick search. The first is a sub-variant of the "Mudtrooper" (aka Imperial Infantry) seen in Solo. They fall under the Special Operations detachment, and the costume reference library can be found here:
Mudtrooper (Medic) CRL
Another, even more obscure option, is the Novatrooper Medic:
Novatrooper Medic CRL
The Mudtrooper may be more what you had in mind, but it's safe to say it will be a more challenging (and expensive) project to take on, mainly because it has a lot more detail parts and components that are unique to that costume.
The Novatrooper, on the other hand, is basically a Shadowtrooper (all black Stormtrooper) with some unique colored trim added to the armor. Shadowtrooper kits aren't as common as white TKs but they're still pretty easy to find. And of course in a pinch you could paint white armor black, since you'd need to paint the trim color anyway. The other benefit of that choice is the construction, fitting, and accessories are all very well documented for the "vanilla" TK variants so you're not having to reinvent the wheel. Novatroopers are also under Spec Ops.
There is a dedicated forum for the Spec Ops detachment (apologies but I don't recall the url offhand). If looking at the Novatrooper option you will also find a lot of useful info on whitearmor.net, including tips such as finding reputable sellers and best practice build techniques, that will be of use whatever uniform you shoot for.
Keep in mind also that everything above assumes you're looking for "screen accurate" (or at least close to it) builds, per 501st requirements. If casual cosplay at Celebration is your only goal you can still use all of this as a guide but simplify and economize on anything that doesn't suit your personal needs or budget. But I assume you'll have a more rewarding experience through researching and doing as much of the work yourself as you can, which is what makes this hobby attractive for most of us in the first place
Best of luck - I know this probably isn't the busiest forum for non-Mando costuming (compared to the RPF amd others), but please let us know how your project comes along!
