So, is it going to be a problem if the total aircraft model is over 17k polys?
I didn't wanted to imply that having this or that polycount would be a problem.
I just wanted to warn you about this diference between polys and tris related to polycount for IL-2.
Polycount for IL-2 models is an issue on its own, where evryone may have a slightly different and personal aproach on it.
Maybe should be good to put an ear to more experience people, have a look on your own to what's in game and how it is done, and then, take whatever fits you right to make your own aproach when doing your models. Don't be afraid to test, because as Loku points out with the R-XIII example, sometimes practice is the best teacher, and you can see better what works best for you. Also, every model/modeller is different, and may have its own particularities.
As example, I aim to below 2k tris for every main mesh, instead of 3k. Usually, most of the parts you will find in a plane won't need as much tris at all, to be good represented, (engines or turrets, being an exception). I first model something without worrying about polycount, just use the polys that you need without wasting them, and then refine the model and add/simplify details where is needed, optimizer tool may be good, but you can be best. Plus trying to optimize manually you will learn how and where you don't need to build so complex in advance.
And if you want a tip, as you mentioned the step before: Most of the time, you can change from mesh to polymesh or viceversa at any time while modelling without any penalty (you don't loose anything on your mesh or model that previously was there)
So if your model currently is a mesh, you can convert it to polymesh.or in reverse, you can convert your polymesh to a mesh. this works back and forth (and the polycount doesn't change, polymeshes are divided in tris also, it's only that their diagonals are hidden).
But, as you only need to have the parts of your model as meshes BEFORE exporting, generally is a good idea to have your parts as polymeshes while you are modelling, because you got a lot more of tools and options for use with polymeshes than just meshes.
Once you are done with the modelling you can switch and convert to mesh before exporting to the game readable .msh format.