Just reporting from some recent observations/tests:
One naked DAZ3d model is around <35k triangles and it looks very smooth, even in close up view. So you won’t need a higher polycount, esp. if the model mesh topology is of perfect quality.
MakeHuman models have >46k triangles, look smooth as well, but the standard topology it comes with, is not as efficient as in DAZ (you can always create your own one as the tool allows for a choice between multiple topologies).
Character hair can look good from <3k tris. The maximum I would go here is up to 10k for a hairstyle. Sims 3 hair is a good example for hair that is low poly and still looks good (in UE as well).
As for vehicles, I’ve seen tractors from farm sim with a tricount of <79k, trailers with ~75k and a combine harvester with a tricount of 100k or even 200k (all an absolute overkill and unnecessary).
Normally vehicles look good with a polycount from 15-30k. Most important are texture quality and good normal maps.
Concerning architectural elements, tools, furniture you should go for as few polys as possible and use normal maps instead as there is no advantage of using more polys than absolutely necessary. (like 1-2k tris for an advanced (special style like vintage) chair or table maximum and <100 tris for a simple realistic table and ~300 tris for a simple realistic chair).
With trees 10k-20k tris should be enough for one tree (at least with TreeIt trees). Farm animals like chicken or pigs can look good with 1k-3k tris.
But you can always use lods, distance based culling and ue culls everything what’s hidden from your view anyway…