You did really good at $50. The reaction to stem shifters really depends on the buyer. To a sophisticated buyer, they will be turned off big time by the stem shifters. But others may like them. I would get some decent six speed clamp on Shimano indexed DT shifters. But the stem shifters on a bike that nice bother me. As a purely financial move, you probably will not make much more than the cost of the shifters by making such a change. Often, less is more, unless the bike is really franken'd up (like the Cannondale T600 I picked up last year, with cheap MTB stem, handlebars and twistgrip shifters haphazardly installed on it). The money I spent putting the right bars, stem and DT shifters was well rewarded.
I would do the regular bearings, grease, and cable replacement. And raise the seat. And a really thorough cleanup.