All you need is the downtube shifter. Get a 6 speed one used on ebay. I'm sure you can find them for under $20. Everything else will work fine with this.
SIS = Shimano Index System or something like that.. but that is all marketing crap. You can use pretty much any shimano rear derailleur for however manynumber of cogs are on your cassette. It is all dependent on the shifter really. Well and the chain/cassette..but i think you already have that covered.