So, I need a new BB. I know that I currently have:
- an ISO/English 68mm shell,
- somewhere around a 122.5mm axle length, [I'll measure more accurately before I buy.]
- a square taper chankset
- a triple crankset.

My first question is what, even just getting a basically matching replacement, does one look for in a BB. There are what seem to be compatible replacements from Shimano's UN26 and UN55 through various offerings from Suntour, Sunlite, FSA, IRD, etc. to expensive ones from Hope, Campy and Velo Orange. Even excluding the expensive ones, is there any practical reason to choose an IRD QB55 over a Shimano UN55 or UN26 besides price, convenience? This is for an inexpensive bike, so cost is an issue.

My second question is what considerations should I make if I'm considering trying a compact double? Should I get a 118mm (2.25mm shorter per side) spindle as a compromise between the double and triple chainlines?

Finally, is there a reason to dump square-taper and move to ISIS or something else as a "future-proofing" option so that if I find cranks I really like I can use them on a more modern frame later? Or is the BB standard smorgasbord on modern frames enough to prevent predictably finding cranks that one can move between frames?