I am way to cheep to have a flip flop hub.
both my fixed gear bikes have old school 5 speed hubs and tubies.
I would just get what my LBS had if i were you.
I ride with my LBS folks
I would get the bike you like the best not the cheepest.
I have fenders on one fixie so it needs dropouts with eyelets to mount fenders or space for fat tires if you cycle cross or ride in bad weather.
make sure it has the right stuff you need for two water bottles.
I don’t like schwinn .
But thats just me.
found a guy on e-bay that will put any size Dawes Lightning SST at your doorstep for $300 - cheap steel bike with a flip-flop that will serve its purpose for this winter. If I use it, I’ll think about upgrading later.