First off... the majority of midi keyboards are USB now and it will plug directly to the computer (not your interface).
Absynth is a great (and unique) synth, but might be a little deep for a first synth. There are alot of tutorials for Absynth though.
There are tons of very good 'free' synths out there. Check this thread for a list to get you started.
http://forum.cockos.com/showthread.php?t=52382.
Enjoy.
