instantssl and rapidssl are quite cheap.
Godaddy are regarded as the cheapest but by all accounts reinforce the adage of buy cheap, buy twice. However unlike adjustable travel forks there are considerably less moving parts. I'd probably give them a go first. You can always go elsewhere.
The 'guff' that the top tier providers add to justify their extra charges will be of no use to you so you can give Verisign and Thawte a miss. Your main concern is that the software validating the cert trusts the CA that issued the cert. For browsers this is generally not a problem but your SSL connections are to and from non-browser software so you might want to check this out.
Oh and make sure your Debian box has the SSL vulnerability http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2008-0166 patched.