Document processing? (TeX/SGML?)

[Jeff Noxon <jeff@planetfall.com>]

I'm looking for the best tool to create a professional-looking document
once, and then render it in the following formats:

HTML
Postscript
PDF
ASCII

I'm not concerned so much about a learning curve as I am about flexibility
and results.  Microsoft Word and other GUI-based products drive me nuts
because they don't give me the control (or stability) I need.

What tools can tackle this kind of chore and what are their relative
merits?  Should I be looking at TeX or SGML tools?  Or should I just stick
with plain HTML?

Naturally, the tools I use should be free and available as part of Debian.

Thanks in advance for your suggestions,

Jeff