As noted, exactly what sort of work you're doing plays a big role in plane selection. That said, I think I should explain the thinking behind my suggestion of the shoulder plane.

Several responses have noted that if you just "cut to the line", you won't need a shoulder plane. This is true. So, how often do you manage to cut precisely to the line for all pieces? Just as importantly, how long does it take you to set up the cuts to that precision? (Remember, he's a power tool shop looking to add hand tools to his repetoire, he's not {currently} going Full Galoot.) One of the most illuminating articles I read once was an interview with a well regarded custom furniture maker. He had a fully equipped shop with large machines. His go to planes? Block, smoother, and shoulder. The block and smoother elicited no notice, being pretty much standard in that respect, but the interviewer asked him about the shoulder plane. His answer? It allowed him to save time. He didn't have to spend nearly as much time dialing in the precision on the machines for tenons. Cut, test, cut, test, etc. He simply set his machine up to make the tenon a bit oversized, and then a few swipes with the shoulder plane and the fit was perfect.

Consider the aggravation savings for the hobbyist. I'm pretty sure that the maker in my example could go through the cut/test routine a lot faster than I could, and he'd probably be a bit more sanguine about screwing up a piece by taking too much off, since he probably has both more stock, and quite possibly some additional blanks already milled. Me, I'd prefer to avoid the aggravation.

So, if you do a lot of mortise and tenon joinery, then the shoulder plane deserves serious consideration. If you don't, then it can be picked up further down the slope.

In truth, you could do 95% of your "bench plane" work with the aformentioned rabbet block plane. Yes, you can flatten a conference table top with it. It will simply take a LOT longer (and more skill) than doing so with a large jointer. Heck, you could do it with a 1/2" chisel. (Basically what a CNC router would do.) The jointer makes the job simpler and/or better and/or quicker. And that's all we're asking our tools to do for us. So what sort of work will you be doing? If you work a lot of large panels and the like, then look at a jointer (#6,7 or 8). If most of your work is small dovetailed jewelry boxes, then a LA block plane and a #3 or #4 smoother are the order of the day, along with a router plane for hinge mortises and such. If you use Dominos, pocket screws, dovetails, or dowels rather than traditional M&T joints, then skip the shoulder plane, but if you do a lot of M&T work, the shoulder plane could become your bestest plane buddy. If you work extensively with plywood, making lots of rabbets and dados and such, then maybe a block plane, rabbet plane and side rabbet or plow plane are your best options. If you get all your wood rough and gnarly, and you build large panels but only have a 6" jointer, then maybe a scrub plane is called for. Each plane has a set of tasks it makes simpler, better or quicker, and there's a lot of overlap between planes. What are the challenges that have arisen that you want to use handplanes to address? Dial in the answer to that question, and you'll dial in the answer to your question.

I was recently re-reading an issue of American Woodworker (a powertool biased magazine), and they covered precisely the question you've asked. Two planes, they suggested a block plane and jack plane. For three planes, they suggest block, smoother and jointer. FWW had an article on the "8 Must Have hand tools" (I think it was eight, coulda been more). Block, smoother, jointer, shoulder were the planes in the list.

Good luck, and don't forget to show pics of your new beasties when you get them.