I´d like to add what periode mono records do you want to play?the ideal mono rig, or course starts with just how serious do you want to get? and how much mono listening will you actually do?
In case you want to dive into shellac you´d need
1) a turntable with variable speed (up to 120 rpm would be best but up to 90 rpm will work for most shellac on the market) - Rek-o-Kut CVS-12 or 16, B-16H, Rondine, L-37 or LP-743 come to mind
2) definetly a phono preamp with variable eq - Hagtech Monogram or Monk Audio come to mind
3) and at least some mono phono cartridges with different stylii sizes what im most cases will point at the vintage ones like GE RPX, Fairchild 225A, Pierre Clement E25B ...
4) with vintage mono phono cartridges you´d better go the vintage tonearm road either. Gray Research 108c and their numerous Japanese copies come to mind.