Interesting challenge.

I know you didn't ask about the wood, but I
think your plan there is more vulnerable than
the paint choice. If the garage gets damp at all
then both MDF and poplar are bad choices. Poplar
is very bad in exterior use. It rots easily. MDF is
just sawdust and glue. It makes a good substrate
for countertops, but it's brittle and vulnerable to
moisture. (I know a lot of plastic-encased cabinet
doors are now MDF. That does help avoid the
drawbacks of moisture vulnerability and easy chipping
of corners, but it's not a good product for painting.)
You didn't mention the case plywood. Birch is also
very vulnerable to moisture. I have to avoid storing
birch plywood scraps in my cellar because they
quickly grow mold and mildew, while fir ply scraps
do not.
So if you're concerned about holding up in the
garage I'd suggest fir or MDO ply, with pine trim.
Fir trim would be even better, but it's a pain to work
with in that usage. It chips and cracks easily due
to the large cells. It's also more expensive.

Enamel just refers to
paint that forms a film. It's better at keeping out
moisture but also peels easier, simply because
there's something to peel. (As opposed to ceiling
paints or stains.) That's why oil stains were so
popular starting in the 80s. They didn't have a
film to peel. But they also didn't last nearly as
long good oil paint over a clean surface. (Now
there are "water base stains", which aren't much
good for much of anything. They're OK for siding
that doesn't get a lot of water exposure, but on
trim or decks they just rub/wear off. ...But they
don't peel.

For a tough finish I'd normally use exterior oil,
like Benj Moore high gloss Impervo. Unfortunately,
they've all been reformulated and I'm not sure
there are any as tough as they used to be.

Most companies make a urethane-reinforced
acrylic paint, which should give you a tough finish.
I don't like those paints for decks/floors because
while they are tough, they're a mess when they
finally do peel. The film is very tough and hard to
scrape. But with inside cabinets I wouldn't worry
about peeling. Being in the cold shouldn't have any
effect, unless your garage is very damp and the
cabinets will tend to grow mildew.

If it were me I'd probably use some leftover interior
oil trim paint if I could, but not because I worry
about the paint holding up in the garage. It's just
that oil paint film is tougher and less permeable than
latex/acrylic, so it's good for the shelves that may
end up having oil, glue, bleach, TSP, or various other
substances spilled on them. You could also use
polyurethane, but that wouldn't look so great on
poplar/plywood, and would show dirt somewhat.