You'll actually lose a little bit of real estate going from dual 15s to a single 18.
15": 1024 * 768 * 2 = 1 572 864
18": 1280 * 1024 = 1 310 720
The difference is relatively negligible, but you may notice it.
As for dual displays on the Mac mini, you may be able to get a USB VGA adapter, similar to this:
http://www.overclockers.com.au/article.php?id=334086
That would allow you to add other displays via USB. I've never used one, esp. on a Mac, so I can't vouch for it either way, but it's something to look into.
HTH.