When beginning to collect vintage Pyrex kitchenware, there is often a tendency to want to acquire every piece encountered. This can lead to a couple of things. One is a smaller bank account. The other can be a lot of glass that quickly becomes uninteresting.
Part of having a successful and rewarding Pyrex collection is forming a strategy. Unless your intent is to become a dealer in kitchenware, buying every piece you find or buying pieces simply because they're at a bargain price is more akin to hoarding than collecting. (Of course, there is something to be said for snatching up those bargains and using the profits from their sale to fund your collecting hobby.) But, if your budget has a limit, think instead about what it is you like most about vintage Pyrex and attempt to base your collection on that.
One option is to focus on completing a set comprised of the best examples you can find of each model made of a particular collection or pattern you like. Don't be discouraged, however, if there is a super-hard to find piece among them. Even if you never find it, you will at least have had the fun of trying.
Another option is to stick with one or a few of the many models of Pyrex ware. If there's a particular type of cooking you favor, there are casseroles, baking pans, and mixing bowls. Or consider instead collecting storage pieces. The versatility of Pyrex ware means most pieces easily serve mutiple functions. And, bear in mind this hobby is one of the few that, if proper care is exercised, allows you to actually use your collection without diminishing its value.
If you find that your interest leans to the broad range of pattern designs, perhaps pick a type of piece and size that you like, and collect that. For example, you could build a collection of only a single size of round casserole, but in as many different patterns and colors as you can find.
Collecting just the promotional pieces alone could also produce an interesting and varied collection. You could, for example, try to collect pieces which were introduced or offered in a year which hold some special meaning for you, such as a birth or marriage.
About Buying Sets
In your search for vintage Pyrex, you'll often find bowls being offered for a single price for a set of three or four. They'll usually be in a nested stack taped together with a criss-cross of clear packing tape, ostensibly to prevent them from being separated. But the tape also precludes your ability to assess the condition of all but the largest bowl. You could risk removing the tape, but, as you know: You break it, you buy it. You could ask the proprietor to remove the tape for you-- and they most likely will-- but don't be surprised to see some scratched up bowls more often than not.
Dealers like selling sets because they believe that's what people want, and also because they believe sets should command a higher price (and thus a higher profit) over and above the individual bowls separately.
People do want sets, but they don't want to pay a premium for pieces of marginal quality, nor do they want to be forced to buy damaged bowls in order to get the one or two in good shape they want. The sets you want and will be the happiest with in the long run are those you assemble yourself, which gives you the option of paying what you want for pieces in the condition you desire. Patience, in this case, is its own reward.
Having said all that, there's a case to be made for having no real strategy at all. While "completing a set" can be challenging and rewarding-- complete sets often being valued more highly than their individual pieces-- there's no harm in collecting pieces just because you like them. And sometimes, buying a piece solely as a memento of the place you found it is rewarding enough.
Finally, bear in mind that a big part of the fun in collecting is in the hunting. Another part is enjoying what you've found. And the last part is passing on your knowledge, experience, enthusiasm, and, ultimately, your collection to others who find the hobby as fascinating as you have. As they say, you can't take it with you.