I have an older home with a bathroom that has poor ventilation which
causes shower steam to coat the walls with moisture.

Same here.

I just went to Lowes, bought a $15 fan and installed it directly above the
bathtub. Wired it in to come on with the light as well. Right now it vents
into the attic; when it warms up outside I will put a soffet vent in,
although my last house (much smaller attic) had one vented into the attic
with no ill effects.

It took about 45 minutes to do. No more steaming of the mirrors or windows,
nor **** smell.