I've been to several mechanics, talked to several friends and posted on
several lists without any kind of definitive solution to my problem,
but i'll post here incase someone has seen something very similar
before.

I have a 91 Toyota MR2 (naturally aspirated) auto transmission with
~80,000mi on it. It's in very good condition for its age, with the
interior and exterior looking practically new and all original factory
parts. Basically, for a 15+ yr old car it's not too bad.

The problem started a couple months ago. I'd be driving along on the
highway and suddenly, the car would start slowing down; imperceptible
at first, but it would get hard to maintain the same speed, and more
gas would have to be given to the point where I would be slamming the
gas down to keep the same speed, and in order not to cause unknown
damage to the car i'd slow down and pull off to the side. While slowing
down I would put it in neutral and rev the engine, and it would rev
without a problem, but put gas and it would feel like it's struggling
to move. Applying the brakes made very little difference.

Pulled off to the side of the road, the car is idling ~800rpms and at a
full stop - with the transmission in drive. I learned I could press the
brakes completely several times, and then the car would creep forward.
I could then drive it like normal but the problem would eventually come
back, at first about 24 hours at a time and now more like an hour at a
time.

I had the fuel filter replaced and it made no difference. I brought it
to a Toyota dealership and they found that the brake fluid was a
milky-white consistency, and proclaimed this was the cause and please
give them $3,000 to replace all my brake parts (hoses, master cylinder,
calipers, probably brake booster too and flush the brake fluid). Since
I don't have that kind of money lying around, I want to find another
way to fix the problem without spending a whole lot. If I can do it in
stages that would be best, so if I have to spend $1500 i'd like to do
that a little bit here, a little bit there when I get the money.

I understand I should flush the brake fluid first, but what next? (I'd
also like to do the fixes myself to save on labor costs, but I am very
much a newbie, so if you happen to have the holy grail of automotive
repair manuals for beginners lying around i'd love to look at it)

I've gotten these suggestions to fix the problem: test the fuel pump's
fuel pressure while driving, replace the cat, rebuild (not replace) the
calipers, replace master cylinder/hoses/calipers, check for vacumm
leaks, replace vacumm hoses, check/replace vacumm sensor.