First, if I'm not mistaken the 686-2 predates 1993 and that means your frame isn't tapped for a mounting rail. This can be done rather easily but a gunsmith or machine shop capable of doing this will charge in the range of 35-50 dollars per hole. Since you'll need to have 3 holes added to the top strap you are looking at 105-150 bucks to have the frame tapped. BTW, I'll attach a PDF file of the mounting pattern at the end of this post, thread size is #6-48 which is a semi custom thread.
Second you'll want to purchase a good quality mounting rail, Weigand has an excellent reputation for a quality rail on this forum. IIRC cost for a rail is in the range of 40-50 bucks.
Third, you'll need a sight and this is an area where you get what you pay for. Spend 50 bucks on a cheap reflex sight and you'll find that it's only suitable for use on a 22 caliber pistol. BTW, I am speaking from experience here, cheap will NOT cut it on anything more powerful than a 22. At the lower end of the price range for a quality reflex sight you have the Burris Fastfire in the range of 240 bucks. Further up the scale are the J Point, the Doctor, and the Trijicon RMR, all around 300 dollars. Top end of the range are the various Eotechs, which range between 500 and 800 dollars.
Can't give you any guidance on the tube type red dots, I've found that the reflex sights work best for me.
Below are 3 of my revolvers equipped with J Point reflex sights on a direct to frame mount. Because it's close to the bore axis this setup eliminates the Convergence created by a large offset. As a result all 3 of thse guns shoot within 1 inch of the dot from 20 feet out to at least 35 yards. It's a very nice setup, however it is NOT cheap, with sight, mount, and shipping I have about 380 dollars in each sight.