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try Postfix if you like headaches 
I use it and didn't really have problems (though that I do not use it as relaying SMTP)
Will lighttpd be enough for the IMCDb, all the more with the (most probably heavier) code of the NextGen? I use it on my computer for "internal" tools and for testing html/php pages and scripts, but feel more "comfortable" when having an Apache for online sites...
lighttpd is much lighter than Apache, taking less CPU (this server runs only on an i3), I don't think it would be quickly limited.
In case at some point it is not enough I could still switch to Nginx.
The new site is more complex, but the database is much better designed (using tables for car makes, models, etc. instead of a rather "plain" system as current with simple text field for each role), so it may reduce the load on the SQL database (currently using much more CPU than the web server).