1. I would rate being able to stand erect under a car I am working on as a priority. Don't scrimp when buying your lift. Make it go as high as you need.
2. I would not. Your shop size is pretty small as it is. I understand the concrete and building size may dictate that.
Totally agree. Air impact guns - I have an Ingersol Rand but there are other good brands - totally blow away electric impacts. I have the larger Milwaukee electric 1/2" drive impact and it struggles / fails at most lug nuts.
A 3/8" electric impact might be fine for an oil pan plug.
I get 5# cans of Mobil #28 grease free at work, so I would use that. The same product is sold at auto parts stores as Mobil #1 grease. It's a red synthetic grease. Good stuff.