I don't want to discourage you from building stuff. Learning is good.
But let me try to adjust your expectations. I've built several over the
last 40 years...from scratch, Built kits, Bought commercial ones.
None are worth a darn
and here are some of the reasons why.

I live 10 miles from Portland, Oregon. Here, there ISN'T a blank spot
on the FM dial you can use for such transmission...at least with
anywhere near legal power output and a receiver you're likely to be
using. If you live in a rural area, this won't be a problem so much.

Pay particular attention to frequency stability. Doesn't do you any
good if it's drifting all over the place. You probably can't retune the
receiver, cause there's a radio station either side of the spot you're
using.



It really depends on a lot of things. I have a Ramsey FM-25 (I think that's
the model) it wasn't cheap (about $120) but the performance is very good.
Frequency is microprocessor controlled with a PLL so stability is excellent.
Transmits in stereo on any frequency in the FM band and the audio quality is
very good. Range for me with the included telescoping whip antenna is about
a 1/4 mile radius around my house. This sounds like more than he needs
though.