Probably the least disruptive rifle sights are the Wilson.
The front sight fits down over the stock bead sight and is epoxied in place.
For extra holding a cross pin can be installed through both sights.
All receiver sights have to be drilled and tapped.
You want to be careful about who you let do this. One of the most basic gunsmithing jobs is drilling and tapping holes for sights and mounts, and you'd think anyone who's in business as a gunsmith could do this easily, but apparently not.
I've been hearing about people getting shotguns back with the holes drilled off-center, extra holes drilled because they got the first ones wrong, and sights sitting off center.
Most factory barrel sights are brazed on, and the red heat required to melt the braze ruins the barrel finish, so it would need to be re-finished.
One good source of ghost ring sights is Wilson Combat.
They'll mount their excellent ghost ring sight set at a decent price.
http://www.wilsoncombat.com/