7 to 11 cassette upgrade or new bike I've recently purchased a 1986 Schwinn Passage. Everything works great, but I'd like to upgrade to STI shifters and the Cassette. I've found a great deal on a groupset, Shimano 105 5800 11 Speed. My current cassette has worn down from the previous owner and one of the teeth on the smallest gear is worn down. I've been looking at Sheldon Brown's articles and I'm unable to figure out what I have.

I'd like to know the best way to go about upgrading my current bike, or if it would be better to purchase a new one instead.
A 11 speed cassette would mean a new hub (probably a new wheel realistically) and likely a new derailleur to match the cable pull of the STI shifters plus the labor (this includes installation of the new cable stops, new cables, re-wrapping the bar after putting on the new brifters, etc.). You will also need to spread the frame to get a modern hub in there, which we can count under labor. 

If you want to keep the bike: You're better off putting on a new 7 speed cassette or freewheel depending on what you have (I'd guess its actually a hub that takes a freewheel) and either living with the current shifters (downtube shifters are lovely and incredibly reliable when you get used to them), going bar-end shifters, going Retroshift or putting a Shimano derailleur and using the 7 speed Shimano Tourney ST-A070 (there is a double &amp; triple version) or compatible Microshift shifters (SB-R472 for doubles/SB-473 for triples). 

If you want to get a new bike: The price of STI shifters and the fact that 7 or 8 speeds is more than enough makes a new bike (or a used bike with STI shifters) a good option unless you're particularly attached to this frame (and everything else is in decent condition or good enough condition that you're willing to bring it to acceptable for you).