money of course is a big factor. I have a small studio myself of about 1500 sf. Beyond the obvious answers of balls, dumbbells and bands, I would purchase again a TRX or equivalent, the efi Power Tower and the Paramount Dual Axis Cable machine. The last two are big ticket items but they are in constant use and are serving me very well with any population.
I have a few pictures of those on my web site www.meltnc.com.