Tahquamenon

I would not fool with the Mossberg ML conversion.

Go with the ML that shoulders and feels the best to you. Don't forget to try the trigger. Many of the lower line ML's if you go that route have terribly heavy triggers.

Look at (not in order):
You are Bill Gates and wish to take up modern muzzleloading:
Ultimate Firearms: BPExpress starting at $1500.00 http://www.ultimatefirearms.com/

Higher End in the 300-800+ range
Knight: Disc Elite or Extreme, Revolution and Vision. Or retro MK-85 Laminate if you find a nice one.
http://www.knightrifles.com/
Thompson/Center: Omega & Encore Arguably the best customer service in the ML business.
http://www.tcarms.com/
Savage: 10MLII
http://www.savagearms.com/
White Rifles: Model 91 & 98 and Thunderbolt
http://www.whiterifles.com/
Pedersoli: Denali
http://www.davide-pedersoli.com/
Austin Halleck: 420 Series. (AH is a smaller firm, but the 420 ML's are nice)
http://www.austinhalleck.com/

Middle End in the 200-400 range
Knight: Wolverine, Bighorn
Thompson/Center: Black Diamond (really a higher end ML, but the design is bit dated). Nice rifles though.
Traditions: Pursuit Pro (not the LT version)
http://www.traditionsfirearms.com/

There are other models as well also from CVA (of which I am not a fan).

The important thing is to select the once that feels the best to you and meets your needs and budget. Follow the manual and always ask questions if you are not sure.

Good Luck!

Tahquamenon