We're kind of dated here. However, the "slightly customized" part and assuming it's used, means you're not covered by their warrantee anyway.
https://montanarifleco.com/warranty-service/
That would not stop me from buying a used rifle though. You can often get a higher end rifle for the same money as a new one from elsewhere. Quite often already scoped too.
"...458 Lott..." Find a reliable source of ammo and/or brass first. Expensive stuff it is too. Midway lists Norma ammo at $94.99 per
10. $9.50 per shot. Federal ammo runs $149.99 per 20. And is in very limited stock.
Norma brass is on sale at $44.99 per 25. Midway has 7 such boxes. There are no other suppliers.