As a newbie I was surfing the site and found this thread, and since hand planes are my "thing", I thought I would post a reply. I could go on for days, but will try to be brief. There's a lot of good stuff in all of the thread replies. The thing that stands out is Andrew's advice about sharpening. Learning how to properly sharpen and tune your plane will take you to a new plateu of abilities. When I first discoverd these "correct" methods, I felt like I had uncovered some deep secret that was hiding in plain sight. It's amazing what these planes (and other old tools) will do when properly sharpened set-up. Your work and skills will improve immensely.
As to what planes do you need, like some of the replies said it's a matter of personal preference. If you're going to do work completely by hand, you will need more of a variety than if you're only going to supplement your power machine tools. I've settled on doing most of the work with power tools but have a pretty full compliment of hand tools right there that I use constantly also. Every woodworker should have a sharp properly tuned Stanley #60 1/2, Miller Falls #56, or similar low angle block plane; this is a must for every woodworker. I keep mine in my pocket when I'm working because I use it so much. As someone mentioned, the #5 is a good size general purpose plane that can be adjusted and used for a wide variety of tasks from roughing to smoothing. So, the bare basics would probably be a #60 1/2 and #5. I have a full set of bench planes in the shop, but since I do most of the work with power tools, I find myself mostly using a #608, #606, #4 1/2 and the Lie-Nielsen #2. If you're going to do a lot of drawer, guides and casework, you'll find some of the specialty planes like a #79, #90, #92, #93 and #98 & #99 to be very useful. I also use the Stanley #45 molding/rabbet plane to make mouldings or to match old mouldings. That's the high spots, but we could easily turn this into a semester long course.
As to what manufacturer to buy, the Lie-Nielsens are the cadilacs and by-far the best. Their quality, adjustability and performance are hard to beat. But they're pricey. I haven't used the Veritas but have headr good things about them. The new Stanleys and Records are probably okay but I don't like them. The best value is probably to seek out some of the older planes. The quality and workmanship in the older planes are great, plus you'll also have a collectible. Stanley, Union, Sargent, Miller Falls are the most well known "old" plane mfgrs, and each of these made quality tools. It should be noted that each of them also made some "low end" and inexpenive model tools that are really not suited to fine woodworking. In the Stanleys, I prefer the type 10 (1907)through type 15 (1936) planes. As someone mentioned in one reply, the type 17's (~1942-1946) "war models" are also good because the castings were heavier on this vintage plane and they perform well. The Stanley Bedrock series are great planes and a good value for your dollar versus Lie-Nielsen. (L-N base their designs on the bedrock) The bedrocks perform well and that's the majority of the planes I have in my user set. Some bedrock models are very collectible and expensive like the #602 (~$600) and #605 1/4 (~+$350) and you probably don't want to use those in the shop.
As to where to get them, some of the dealer guys mentioned in the replies are good sources. Ebay is good if you know what you're looking for (and looking out for). Watch for cracked castings, missing parts, etc. There's a bunch of good guys on ebay that are reputable like "oklatrader", "wanderso1", etc. If there's something you're looking for, I may have it, especially in the bench planes size #2 through #8. I'm pretty sure I've crossed the line from "collecting" to "compulsive-obsesive disorder".