Condition and originality are paramount in a collectable gun, your rear site blade and screws have been changed from the original SS to black replacements, while this is reversible (if u can source originals) the worse news is someone has added a red ramp at some point which is not so easy to reverse although your fs blade is actually pinned to the ramp then polished flush ( look closely you will see the pin) but again good luck finding a replacement SS blade.
If it were a very early sample with a K serial prefix (like K999,999) it might be worth looking into restoring, but with a floating K serial not so much IMO.
Your Model 66 will make a fine shooter but avoid light fast .357 110 grain mag loads, stick to factory 158 grain 357 or any .38 Special.
Any mechanically solid M66 is easily worth $500 to someone wants to shoot it and doesnt collect or want to pay collector prices.
IMO a LNIB 70-75 4" Model 66 is at least a $1000 gun, one with a K prefix serial add another $50%