Need To Know More About My Omega Seamaster De Ville Watch

Hello!
I bought a box of watches at an auction a week ago and this was among them. I have no clue about the last time it was serviced, when the battery was replaced, or when/where it was originally purchased. All I know is that it was still ticking when I picked up out the box to take a closer look at it, but I believe the battery is running low and the date feature has not changed. When I turn the little knob the time changes, obviously, but I can't change the date (if that's how you even change it). Please forgive my lack of correct watch term lingo. The inside of the face reads "Omega Automatic" "Seamaster De Ville" "Swiss Made". The inside also appears to be scratch free. The glass on the outside does have scratches, but no scuffs. On the outside edge of the watch, just under where the band meets the watch face, an imprint reads "14 kt. gold filled". I have not attempted to take the back off and see what's going on inside (I do not feel comfortable doing so). The band is a stretchy band that reads "Speidel U.S.A". I am looking for information about it's worth, an estimated guess of what it may cost to have it serviced, and any additional information you would like to share. I will take it to a jeweler for service/cleaning if it will be worth it in the end. I hope I have conveyed enough information, but please don't hesitate to ask for more. Thank you!!
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Hi. There is no battery in that watch. It's a mid 60's self-winding mechanical unit inside. It definitely needs to be serviced to run properly, and probably needs parts if the date won't change. You might be looking at a $200 repair / service cost - or more.

Thank you for the information!