There are a lot of different ways to set things up, and it is possible to work around less than ideal equipment, but it's definitely handy to have a three vessel setup (at least - my ideal is to have 4, the 4th being a water building vessel, which would be pumped to the HLT). I'm actually rebuilding and adapting to my brew partner, who owned most of our equipment, leaving town to run a brewpub in Nebraska. Even at it's peak our setup was pretty simple: an HLT, a mash tun(MT, sometimes called a mash-lauter tun, or MLT), and a boil pot (BP). Our HLT was a 10 Gallon pot with a ball valve, our MT and BP were keggles with weldless ball valves and a grate in the bottom, with an elbowed pickup tube that went down through the grate into the little hollow at the bottom of the sanke kegs - this setup is available as a kit that costs ~$70. With this setup you can drain from your MT into your BP as you sparge from your HLT. If you get the MT draining into the BP at the same rate as you add sparge water from the HLT, you can keep the water at the same level, hopefully just above your grain bed. How many burners you have can come into play as well. The advantage of using a keg as a MT is that you can directly heat it in order to move from one temp rest to another, but you need at least 2 burners to do that. We would actually have the BP and the MT on burners (this also allows you to start heating the wort in the BP while still sparging), then heat water in smaller pots on the stove and dump them into the HLT, so a third burner would have come in handy. You can do exactly the same thing using a non-heatable MT, you just have to be a more precise with your hot water additions - I like the flexibility of a metal MT. The bottom line is that you have to think through the whole process, and the ways that each step can be accomplished. It's almost a good thing that most homebrewers have to patch together their equipment as they go due to financial constraints, because you really start figuring things out when you start brewing. I'd advise you to read and study enough to have the basic steps in mind, know the equipment alternatives (for example, one quick and dirty way to put together either a MT or a BP is to just screw a bazooka tube screen into a ball valve equipped stainless steel pot, with the size depending on the batch size. I've brewed quite a few batches that way. It works, and the tube is <$20. It does make directly heating your mash less practical due to potential scorching, since the grain sits on the bottom of the pot.), then brew a few batches. The various problems, possibilities, methods and choices will become clearer. Good luck.
