Yep ..you do that!. Very much adviceable!.

Bore-doublerifles are becomming scarce, and attractive to collectors/doublerifle-buffs these days.
The Manton 10 bore I bought last year for 15K would cost me even more today if a similar gun were to be found in same condition . Realistic price for a good, strong 8 bore double could easely go into the 20-30K figures. Even though you want a "shooter", you still need a gun that is strong, tight, good rifling, "on face", not too pitted internally/externally.
You shall also deside what type of borerifle you want. Some can shoot roundballs only, some are made for projectiles(faster twist), papercased gun (.835cal) or a brass cased gun (.875cal)??. Believe me..there are a price difference in all these things I mention here.

Look around a lot, and never be affraid of asking too much, and get a feeling of the market. You might get lucky and envy all of us here...hahahaha