I too have a 40yo 7" Makita. Takes both hands to carry it around but made to take a beating. The type of work you are planning is 1)crappy, so you want it to go as fast as possible and 2)Very hard on the grinder.
I have several air grinders, the big 7", several 4.5" grinders and a cordless Dewalt. The cordless is good for quick (15min) jobs away from a 120v plug. The air grinders are more for medium load, non-stop, hours of continuous running (they are light and generate no heat themselves), the 4.5" are good for heavy metal or paint removal in a small area and the big 7" is likely my first choice for much of what you are describing.
You don't want to skimp here.
