Jeff, if you are really serious about getting to 20 micron, FDM/FFF printers may not be what you need to be looking at, very few are capable of that resolution, regardless of marketing. You may want to look more into laser SLA and DLP printers. That WILL get you the resolution that you are looking for, and there are resins available that certainly meet your durability requirements.

Two specifically that I would suggest you look at are the Moai, on Kickstarter (within your budget); and the G3D T-1000 on Indiegogo (a bit over your budget). While these are both crowdfunding projects, they are both extremely likely to ship, both have shown that they are ready and the campaigns are more just for funding to ramp up production. There are others you could look at as well, and any of them would certainly last for the duration of your project and a lot longer.

Additionally, your concerns about the biodegradability of PLA are kind of out of proportion. While it is made from starch and it is biodegradable, it is not on a short term scale by any means, and finishing such as paint is going to prolong that considerably.