modernizing shifters
Shimano never made a derailleur or shifter called "515", as far as I can tell. Perhaps you mean "SIS"? SIS stands (I think) for Shimano Integrated Shifting, which is indexed shifting.

The fact that you've only got five cogs in back suggests that you've got an old, low-end bike. You might find that upgrading your shifters will exceed the total value of the bike. I also don't know whether Shimano's low-end derailleurs are compatible with their brifters for indexed shifting. I'd take it by a bike shop and ask for an opinion.