Buying used doubles - rifles or shotguns - can be a hugely expensive education. I am fortunate to own a number, but getting there was 40 years of hard-earned and often very frustrating experience. I would guess of all the used or vintage doubles I handle in a given year - several dozen - at least 60-70% have some issue which will cost serious investment to correct. Barrels off face or out of proof; barrels re-chambered to another bullet or cartridge length; amateur "repairs" to actions, ejectors, or worst of all triggers, barrels re-soddered but not regulated, etc, etc, etc. If you are new to these guns and rifles, the cheapest investment you can make is in a buying service.
http://www.vintageguns.co.uk/company/diggory-hadoke Colleagues have used Dig's services with great success. At least buy with an inspection period and have it looked over closely by a gun maker who really knows sxs's.