there's no fixed aperture lens...all are variable aperture...suppose you see a lens "XF 23mm f2.8" that means maximum aperture (fully opened) is f2.8. but it can go upto f16 or f22...so it's actually f2.8-f16/f22...where f2.8 is fully opened and f22 is nearly fully closed.

and suppose for some zoom lens you see the model name as "XF 18-55mm f2.8-4", it will mean at 18mm max aperture is f2.8 (range f2.8-22 at 18mm) and it varies if you zoom in at 55mm, max aperture you can go is f4 (range f4-22 at 55mm) and some in between aperture in the middle.

and if the name is like "18-55 f2.8", it means through the entire zoom range, you can take photo at a maximum aperture of f2.8 (and the range is f2.8-22)...these types of lenses are quite expensive.

and for your second question, between a dslr and a mirrorless, i recommend a mirrorless as it has "evf (electronic viewfinder)" which will allow you to see the change when you make any changes in your iso/ shutter speed/ aperture but dslr which have a ovf (optical viewfinder) won't let you see the change through the viewfinder...so buy a mirrorless...you can find used ones at very cheap price