dunno about purchasing books from other sources via wifi since i don't buy DRM'd material...
have been using ssh and a terminal app on my 'upgraded' Nook, and can easily access my iMac
but since i only transfer books via USB (much easier/faster) i haven't tried wifi transfer... you could try:
http://www.lysesoft.com/products/andftp/
(note: i have not tried this app, but will at some point)
btw, you may want to read up on how to register your Nook Touch (must have an U.S.-based IP, IIRC?) ... once registered, and with your gmail account, 'upgrading' is simple and easy
i quite enjoy my Nook Touch now; it works as advertised with few problems, and a number of useful apps and games are only a 'touch' away via Go Launcher EX (the 'droid UI)
hth