I would suggest an inexpensive electret pzm type mic as having the best
overall performance parameters as to reach and intelligibility. I'm not
convinced the gear needs to be expensive, just that it have enough
headroom to handle frequency shaping and the ability to comperess the
final signal. When I was a child hearing aids were box affairs that were
carried on a belt with an earpiece going from it and the mic was in the
box, did I mention it was tube powered. Chip amps that fit in a radio the
size of a pack of cards are common and could do the job nicely with some
sensitive headphones which are often in the 90 + db range. Doing the
frequency shaping using dsp in the research stage would be good with the
final assembly done using the amp chip with a compression ic and a
network. It might be useful to consider telephone technology where years
of research went into finding the amount of compression and frequency
shaping that provides the best intelligibility. I fear your presumption
about hifi gear and best intelligibility are two mostly unrelated factors
with divergent goals.