I have metal cutting band saw and difference to wood band saw is very much lower band speed. If you don't do production work you can opt for low speed for iron metals and use that speed to cut all metals (just a little slower and a little less productive, not biggie for us hobbyists).
Therefore I recommend jack shaft and extra pulleys to reduce the speed to bearable level. You don't need VFD, unless you really insist, no point of adjusting the speed down with VFD in stead of jack shaft reducer.
One thing that you might consider is that for wood coolant is not used (except logging saws), but often for metal, least cutting oil and brush on cutting saw.
About the guides....the material should not be plastic. It might of fiber material, sintered metal, bronze or that sort of stuff. Could work on metal cutting bandsaws too, but needs adjustment, cleaning and it does wear out. Not a game changer if you use gently and not all the time. Personally I would do the speed conversion firs and then see how much of an issue this is. I would plan of putting bearings on back of the blade and use those orginal stationary guides on side, if it is easy to modify that way. You don't really want to force the piece against the blade anyway with too much force, the blade must cut freely and carry the chips away.
You need the metal blade...have you checked if there are ready made blades for that width/length? Wood blade probably will work for brass and aluminium when used at lower speed.