I started with an old road bike.
http://www.biketcba.org/TRICORR/proj.../junkbike.html
I would recommend using something higher-end than a dept-store tank as a starting point; you'll end up with a much better bike for all your effort.
You can make your own seat, but buying a fiberglass hardshell at $100 isn't a bad way to go. The WISIL site has instructions on building a seat or a mount.
http://www.recumbents.com/wisil.htm