The value depends largely on the model of the Sako you are looking at. Almost any Sako in good condition will bring $700 - $800. More if it is a true collector. Your best bet for current value is to check with the Sako Collectors Club:
Sako Collectors Club Forum
You can also check on the various websites like gunbrokers.com to see if anything similar has sold recently.
Sako's have traditionally been great shooters right out of the box. While this 243 may turn out to be a great rifle and a great buy for you, it may not be a great long range rifle (if that is your goal). Rifles of 20 years ago typically had barrels with slower twists that may not be able to stabilize the longer, heavier bullets used today for long range shooting.
Still sounds like a good find. Good luck to you.