Originally Posted by vitrael
I don't know about a kit, check your LHBS, but...
If you already have everything you need for extract brewing, all you really need is a mash tun (picnic coolers work great), lauter tun (2 stacked buckets, top one drilled with holes, bottom one with a spigot for racking) and a kettle big enough to do full boils and you should be good to go. Oh, and a very good thermometer--you will want fast, accurate reads.
There should be some tutorials on DIY mash and lauter tuns if you search, elsewise Joy of Homebrewing has a good illustration of how to use the supplies I mentioned.
You don't need separate mash and lauter tuns! But, yes, coolers do work great for combination mash/lauter tuns. If you want to go the kit route for that, Morebeer and others sell already converted coolers with false bottoms. But you can make one much cheaper yourself.
You can get away with just one pot for heating water and boiling wort. For 5 gallon batches, I would go with at least an 8 gallon pot.
Although not absolutely necessary, you will probably want something to use as a hot liquor tank (HLT). This will hold your heated sparge water, allowing your boil pot to accept the runnings while you sparge. Again a cooler will work just fine for this, no need to spend money on another pot.
Everthing else you probably already have.
Take a look at some three tier systems to see how this flows without pumps. No need to build anything fancy, you just want to put the vessels so that the HLT can gravity flow to the mash tun, which gravity flows to the boil kettle. The most difficult part of this process is getting the hot water from the Kettle to the HLT.