The most common AG setup involves a hot liquor tank (hot water), a mash tun / lautering tun, and a boil kettlle. Take a look at the beer sculptures on this forum in the gallery and at More Beer or SABCO and you'll see that this arrangement is very common. The gravity-fed systems typically have the various kettles in a tree configuration with the HLT at the top, the the MLT, then the boil kettle. The systems that use a pump typically have all three kegs or kettles at the same height on a stand, with the pump replacing gravity for movement of water and wort. There are as many variations as there are ingenious people who figure out what will work for them.
You need a minimum of two vessels. A hot liquor tank to supply clean hot water for mashing and lautering, and a mash tun / lauter tun. If you do a single batch mash infusion, that's all you need. You can heat the water in the HLT, drain it into your MLT, wait an hour or an hour and a half, then drain back into your HLT, which now becomes your boil kettle.
To do fly sparging or multiple batch sparging, you really need three vessels. This is because you'll have to have something to drain off the mash in, then you still need hot water coming from your HLT to sparge with, whether fly or batch.
The good thing is that you can get away with one boil kettle and two ice coolers. Many people use two 5 or 10 gallon cylindrical water coolers, Igloo Ice Cubes, or other rectangular 48 quart or larger ice chests.
I did an all grain brew last weekend using my 13.2 gallon Lowenbrau kettle for both the HLT and the boil kettle, and two 48 quart Igloo Ice Cube coolers. I had the modified keg on top of a propane burner from a turkey fryer that was sitting on top of a sturdy table. I heated the water to 169 in there, then drained it by gravity through a clear vinyl hose connected to the ball valve into the first 48 quart Ice Cube that I set up as a mash tun. I'd replaced the standard spigot in the Ice Cube with a Kewler Kit weldless fitting that has 1/2 inch NPT threaded connections on either side. On the outside, I fitted a ball vavle, and on the inside a barb fitting to connect to the manifold I made for slowly draining the wort. When it was time, I drained the wort into another Ice Cube at the rate of about a quart a minute. I then batch sparged with another few gallons of water and drained again to get all the sugars out that I could.
So now my wort was in the second Ice Cube. I poured it into my kettle, topped up with the more water to make 6.5 gallons, and proceeded with my 90 minute boil.
I could not have managed that with only two vessels. Happily, the two Igloo Ice Cubes were very cheap.
Having said all that, I do think a SABCO or similar keg is the best way to go for the boil kettle. You can do 10 gallon batches in it. It is extremely durable, easy to clean, and easy to use. To do batch sparging you don't really need the false bottom or the interior siphon that connects to it, because you are just going to use it for HLT and boil kettle, while you use an ice chest with a manifold for the mash.
I plan to get two more kegs eventually so I can have a complete system using kegs, but that will be a long time from now.
Caution: I'm quite new at all this. If the wiser and more experienced brewers on this forum contradict me, you should go with their advice. I would.