What are the limitations when baking textures?

Question

3D

1) What is advisable to do with the help of baking, and what is better to "sculpt" manually? How to understand the limit of "possibility" of baking, so as not to waste/reduce time on samples?
2) What are some approaches/thoughts to save polygons in terms of baking? For example, I am creating a model for sale on stocks, I want to make it "game ready". Does it make sense to bake to turn 300 polygons into 100? And 300 to 250? And 10k in 8k?
3) How close should LP and HP be in terms of geometry?

Thanks in advance.