In the 50's and 60's a block plane was often the only plane that the home handyman would own. There was a race to the bottom as manufacturers vied with each other to offer the cheapest possible block plane. Consequently, some of the worst hand tools I've ever encountered are these entry level block planes. Stay well clear of them, you might get lucky, more likely it'll be money down the toilet.
Another thing to think about is that block planes get used in lots of different ways. If you plan on using yours like a miniature, one handed plane, used chiefly on long grain for tasks like chamfering or knocking off the arris, then you've got a very wide selection to choose from. Most decent block planes will give you satisfactory service. But if you will chiefly use your block plane for dealing with end grain or for other difficult jobs like cleaning up the edges of plywood, then there's a stronger case for one of the specialist steels like A2 or, even better, the new PM-V11 from Veritas. For most woodworking in temperate zone timbers I'm not entirely convinced of the advantages of these steels, but block planes are often asked to perform at the limits of tool steel resilience, where specialist steels are undoubtedly more useful.
One final point, tools are useless unless you can keep them sharp. But block planes are often awkward tools to sharpen. They're sometimes a bit small to conveniently hand hold, especially if you go for a smaller "apron" style of block plane. And because they're often low angle they need a longer projection from a honing guide, which then introduces more complexity. Veritas are particularly difficult in this respect because they tend to have a tapered blade design, which leaves less material available for side gripping honing guides. There are solutions to all these problems, just be aware of them before you buy.
