Its possible, but probably more complicated than you'd fancy.
Linux Box (or windows i suppose)
Big HDD or a few.
ADSL Modem PCI card (as to avoid the extra box you don't want)
Lots of Network Cards (1 per machine you wish to connect)
Wireless PCI card (to provide wifi peer to peer link).
Install yourself a firewall type linux distro (or windows with some software firewall) and the ftp server is easy enough.
Personally,
I'd buy a decent ADSL Modem Router with wireless to cover most of your bases, and perhaps one which has a built in print server. Then get a nice little NAS (It'll have FTP if you buy something like a lacie) with ethernet and plug her in.
Cheaper, less complicated, and it'll look prettier..