First, welcome to BYC
If you think there is *any* possibility of adding more, I'd either build just a (very strong predatorproof) tractor for now, or a large enough coop to house more chickens than it currently seems likely that you'll have. Otherwise I can almost guarantee you'll run out of room
People on this board mostly give their hens 4 sq ft (that's 2'x2', or the equivalent) per chicken indoors, plus 6-10 sq ft per chicken of outdoor run.
OTOH more is definitely better, especially if you are in a climate where there may be long stretches of weather when they just don't want to be outside.
(I go 15 sq ft per chicken indoors plus run, and they really DO seem happier and mroe relaxed and more 'natural' that way than in smaller quarters, IME)
You'll only need one nestbox per 3-4 hens, though
and a foot, or not quite, of roost space per hen.
Have fun,
Pat