If you have a more primitive weapon rule, the T/C Hawken or Renegade sidlelock in .50 cal are excellent rifles. There are also several other decent sidelock rifles in .50 available from other mfg's.

Perhaps your state will allow an in-line ML with a bolt or plunger type design that uses a #11 cap?

Don't forget about at least $100 budget (Likely more) for essential accessories (Powder, primers, cleaning and maintenance supplies and tools, projectiles, and so on)
See the thread "ML accessories". I've met several folks that were somewhat surprised when they did not budget for the "essentials" beforehand.

Also, if optics are desired allow at least $150-200 for scope, mount, rings, installation and boresighting.