A single club buy must be a Wedge IMHO - get a quality shot to the flag!
If "one club only" is to be maintained "forever", I' want to discuss if not to prefer a 75/80 yds club or even a 100yds club - bigger range for good shots - think of chips and flops.
For a budget one-by-one extention, I'd propose a 3WD (replacing the Starter driver) and a second wedge.