My Experience:
The basic service changes the battery and tests or changes the seals. Battery needed every 2 - 3 years. I found Renata lasts a bit longer than Varta but both work the same while they do and freshness / exp date counts more than the battery differences.
The full service checks and polishes/restores everything incl new battery, seals etc. but is a significant cost even as a % that still doesn't cover you for deep scratches and chips etc.
As they get a bit older (mine is now 20!) you may care/dare less about swimming wearing it and I found after 10 years I decided I was brave enough to (quite easily) remove the back and change the battery ( at your own risk !!). Do not assume it is in any way waterproof thereafter.
Check details on your particular watch e.g. The older 95 - 99 models used a 399 battery. If it shows rubbish on the display when the new battery is in it you may need to remove the battery for a few minutes and try again. (or on some there is an alternative routine of something like selecting 2x blank digital displays, pulling out the crown and then clicking back and pressing in for 10 seconds to reset YMMV)
Again you'd need to google your particular model as there have been many variations of this classic watch
Should you decide to sell it an official service immediately before may well add value