All 686's that are -4 and up come from the factory with the top strap drilled and tapped for a scope mount. It's under the rear sight, so all you have to do is unscrew the rear sight assy, and put on the mount. So it's unlikely that there will be any problem just reinstalling a rear sight, since it was made this way. Ask the seller if they have the rear sight with it in the box, etc. If it has the rear sight, then it's no drama there at all.
The -6 has some MIM parts and the key lock. -6 production started in 2001. I can't tell for sure from the pic, but those don't look like MIM hammer or trigger. Given the trigger stop screw and the TK mark, it may very well had some really nice action work done.
Some folks don't like the key lock and the absence of the rear sight is a value downer as well. TK moon clip work is a plus. IIRC it still works like normal without the moon clips, but you have the ability to take moon clips. That set-up might indicate it was used in competition, which may be a plus or minus. It could mean it has lots of miles, or it could mean it has some valuable work done to it and has been taken care of. Only looking at it would tell you.
$400 is a good price. I'd buy it if I ran across it in a local establishment. I don't mind a 6" barrel either. I wouldn't buy it if I didn't want a 6" barrel. There's no point in spending money on something you don't want.