Miniatures Advice

For those of you who run campaigns with custom terrain and miniatures: do you have any advice on using them?

It can definitely be hard laying out a whole dungeon so on average how much do you lay out at a time? Do you use line of sight before placing miniatures? Do you remove minis or do you leave them on the board when they die? (This one is more niche but like what if they heal lol)

Any advice or tips and tricks on using them that has made you life easier or ways to make/buy quality pieces would be greatly appreciated.

Finally if you buy terrain or minis how do you pay for them? Does the party have a fund or do you the dm just get stuck with the bill?