I think the biggest gamble is bore condition. I felt like I'd won the lottery when my William Moore showed up with no pitting and only light surface rust inside the bore.
Next to worry about is frozen nipples (don't laugh). Getting them out can be an ordeal.
Getting the ribs or barrels re-soldered would be tough, most smiths have never done this and few are qualified for this job. If a set of barrels needed Any repair in this area I'd pass on them.
I'm sure others will have good information as well.
Along with the good points Brother ETerry mentioned, one must think of WHY the barrels would be separate from the rest of the gun?
There may also be a ding or bulge in one or more barrels. A ding can be fixed, but not a bulge - other than to saw off the barrels behind it. Further, the owner may not even realize either of these things are wrong with the barrels. Worse still, there may be a small crack or split in one or more barrels and again something the seller may not even notice. Those can't be fixed except at exorbitant cost.
Separate double ML barrels often if not usually have at least one, if not both nipples broken and or frozen in place and they are often a PITA to remove AND then you may be stuck with an unusual size of nipple thread that is no longer made. In that case, you may be able to use a larger thread nipple, but you will have to tap the holes again and hope the metal around it is also not rusted out. Worse case scenario is you would have to custom make replacement nipples or have them made.
Can you tell I've looked at a fair share of separate double barrels and complete ML guns with double barrels?
In my early years of working guns, I can't tell you how many ML double barrel "family heirloom" guns I've looked at that were mostly complete or complete, but were not worth anywhere close to what it would cost to bring them back to shooting order. Even if purchased for very little money and one didn't mind the time it would take to fix it, most weren't worth it.
Bottom line, there is NO way I would buy a set of double barrels without being able to inspect them closely and even then, most would not be worth buying.
Gus