SD + CC + Budget = 9mm
In my book the variables you list make for a pretty clean decision...
On the CALIBER / AMMO QUESTION:
9mm is a GREAT Weapon Caliber for Self Defense and Conceal Carry, AND the Ammo itself is cheap enough to shoot with Great Frequency - Academy Sports sells CCI Blazer 9mm for Less Than $4.00 per 50 rounds. Never had any problems with the CCI loads (although some ranges do not allow CCI use to prevent contamination their brass - CCI uses aluminum casings). Winchester White Box (WWB) 9mm is Less than $5 per 50 virtually Everywhere.
On the WEAPON QUESTION:
For a NEW 9mm - and a Great Value to boot - the S&W SIGMA Series in 9mm - the SIGMA SW9VE is an Awesome Weapon (16+1 Mag Cap, Accurate, Lightweight, Never a FTF or malfunction) for an INCERDIBLE Price - Brand New with extra mag is Less Than $300.00.
Another GREAT Weapon in the Budget 9mm family with a Great History and high capacity is the Kel-Tec P11. Again, less than $300 Brand New and a GREAT Lightweight Workhorse!
Of course, both of these questions will always receive very subjective answers - Great Fodder for Great Debate!
Just my opinion - Good luck in your qwest - let us know how you end up!