it's the same machine. There was one on the monoprice stall at lat year's tct - they'd just stuck tape over the flashforge logo :-)

Yes can use any brand filament and yes it's a lot more economical to use an external spool holder.
In the uk flashforge sell the 5/600gm spools for about ?11 and the 1kg rolls for ?14.
Kind of a no brainer :-)

as it's a printer with an enclosed print volume, you can pretty much print anything. You might struggle with polycarbonate - but that's just because it needs a fairly serious hotend. Most other things will be fine :-)

Most of the monoprice machines are reasonable.
But will most likely need some minor mods to get the best from them,.

I've got the mini delta. yesterday I made the bowden tube shorter and cut the extruder spring shorter.

It now prints flexible filaments that it didn't before. I have a 0.25mm nozzle on mine - so it's used for small highly detaild or fiddly prints the other machines don't like.
But monoprice just buy in printers and rebadge, and generally they buy from quality manufacturers.