Looks like your only restriction is no scopes.

I would suggest a Optima V2 in nitride finish. Those are under $350.
https://www.muzzle-loaders.com/cvatm...en-sights.html

Next step up is the Accura V2 in nitride for under $500
https://www.muzzle-loaders.com/cva-a...-pr3122sn.html

Knight rifle still has Extremes on sale for $500 and with shipping its $540-$560. You really need the bare primer upgrade for these though and thats another $50ish shipped. Extremes are very well made with a Timney trigger also.
https://www.muzzleloaders.com/produc...-muzzleloader/

You would need this plug and adapter to make it a really good rifle. These bare primer conversion parts are top shelf. Ive used quite a few of them.
https://sskfirearms.lehighdefense.co...product_id=259
https://sskfirearms.lehighdefense.co...product_id=258