Lot size? in a park it’s all over the place in my area but I think singlewides should be 6-8 per acre. doubles 4-5 per.
Backrounds? I use mrlandlord.com credit service. inexpensive and fast.
Skirting? As with every other thing in this biz it’s negotiable. I’m way to cheap at this time to pay for skirting installation (besides my wife Karen helps with that job!) materials cost for a singlewide 550-700
In NY most homes are vinyl it’s the cheapest and easiest.
One problem we have here is snow/ice slides off the roof and falls against the skirting and caves it in then the pipes freeze from the wind (fun-fun)