Just get a Sora RD-3500-GS and an 11-32 cassette:

Shimano 9 Spd Deore HG50 Cassette, Cassettes, CASSETTES MTB

Shimano Sora 3500 9 Speed Rear Derailleur, DERAILLEURS REAR

Should be doable for under $50 in parts, unless a new chain is needed in which case it's probably under $60 total.

If that's not good enough, then go to a compact crankset. Changing the shifters in order to get a triple is not going to be necessary, especially if a 34-tooth cassette can fit under the Sora RD.