Is a single speed folding bike good for hills?

Not really. With one gear you push the same ratio uphill and spin it out downhill. It is fine on flat city streets and gentle slopes, but steep climbs will be hard.

How is a 24-inch folder different from a 20-inch one?

The larger wheels roll over bumps and cover ground more easily, and the bike feels more stable at speed. The trade-off is a bigger folded package that is harder to carry on crowded transit.

Is a steel frame a problem?

Steel is strong and soaks up road vibration, which makes for a comfortable ride. It is heavier than aluminum, so lifting the folded bike up stairs takes more effort.

Do I need to maintain a single speed bike?

Less than a geared bike. There is no derailleur or cassette to tune, so upkeep is mostly tire pressure, chain lubrication and brake checks.

Who is this bike designed for?

It is built around a women's frame design, so riders who find a standard top tube awkward to step over may prefer it. Anyone doing short, flat trips will get the most from it.