Depends on what you want from your hobby mate. A bike of that age and condition, as you've found out, will cost quite a bit to get into "show" condition, but it really depends on what you want to achieve. Do you want to continue to learn a good skill set that allows you to tackle issues as they arise, do you want to buy cheap and turn around for profit or do you just want to ride? If you tinker AND ride you will find a lot of riding time can be eaten up with tinker time, although you can pre-empt maintenance during off peak times.
I'd personally enjoy her for what she is, and work on eliminating that flat spot. Ride her in the spring, then decide what you want to do next. No point even considering selling her this time of year.