I have been down this road with my studio very early on. Thousands wasted on foliage we will never use, simply based off of the fact everyone seems to think 50k poly is an acceptable “game ready” single tree.
Honestly, this is going to fall on a lot of variables, more than just “what works?”. Primary question is what will work FOR YOU. Now, there is a fine line between poly count and details. I own dang near all trees on the market place, and, i ended up using the ones from the Neighborhood Modular pack because they work amazingly well. Any of the trees, that i have seen, that come in most “packs” are kite demo trees. Do not use those. Stay away from them. Super high poly and a real pain to work with. Sure they are pretty. I mean, unless you want to spend a couple months with optimizing the trees only.
It comes down to the fine line of pretty and functional. How many trees? How much foliage? How big of a space and how many actors? Are you willing/able to use billboards on the culling? Are you willing to reduce the look/feel down to looking like a cheap 1990s game jsut to have up-close 8k trees? I love the “Redwoods” and the “Broad leaf” setup from M<something> United. They make amazing assets. But 30k+ poly per actor is a joke when they say “game ready”. If your map is like 1km x 1km and has a handful of trees, you will be all set! Imagine having 10k trees on your map. Thats a SUPER low number, depending on the game type. 300 million additional polys will destroy your FPS. For a tree.
Again, it falls to what you are willing to lose to gain visual clarity, or, what you are willing to lose to gain overall FPS and stability/smoothness.
Speed tree has good poly counts. Really thats the best “bang for your buck”. Stay away from anything “ArchVis” as the poly counts are outrageous in that stuff.
EDIT
Materials and textures, make yourself. Google some images and add them to the material and you will have exactly what you need for free.
Landscapes stay away from buying, unless you can “test” it and see if its EXACTLY what you want/need. I honestly would ask if someone can toss you a height map. Sounds like with that many biomes, your going to need something pretty extensive tho.