Nearly any indexed friction rear derailleur will work with your friction shifters. Shimano and SunTour derailleurs from the mid-80's are plentyful on ebay and can be had pretty cheap.

Indexed shifters can often also be had fairly cheap, but ensure you're getting a complete and working set. Some early Shimano levers (6speed in my experience) often came in many many small parts that had to be put together in the right order. Easier said than done. Also you have to make sure that you have the right type of brazed on bosses for the shifters. Shimano SIS shifters require a specific design of boss to mount to.

Lastly - if you decide to get indexed shifting, know that the rear derailleur you buy will need to be compatable with it, and the freewheel too (# of speeds and brand). Actually, that should be the first thing considered. In any case, decide if you want indexed shifting, then replace the derailleur and possibly freewheel after that decision is made.

Others will likely be able to tell you more of what is possible with your specific bike, as I am unfimilar with your bike. Good luck.