I don't see the need when you have a KSM44. How about a Beyer M69, or save up a little longer for a M88 or Senn MD421? You'll like those on drums quite a lot, and it's a different sound for vocals.
I would think a "matched-pair" of small diaphragm condensors would be a nice addition for recording acoustic guitar and piano in stereo, and they could double as drum overheads should you decide to do that later. You won't be able to get a pair for $130 new, but possibly could find them used. MXL 603s, Oktava MK 012, & Studio Projects C4 are all viable options for not too much $$ ($200-$300 for the pair).
I agree with mshilarious that a single 603 would not add much. Some people like SDCs for acoustic more than LDCs, but it is largely preference and not a must. If you are on a tight budget, try the Behringer ECM 8000 reference mic. It is only $50, omni pattern, and sounds very flat on acoustic. Though not often great on acoustic guitar, it also never hurts to have some dynamics around (SM57).