It all depends on what you are doing. In my opinion, the cheap kits from the big name suppliers will work fine bumping up a ding or working a weld enough to accept bondo. I bought one of the cheap sets and then filed, sanded and polished all the surfaces, including some of the backs and sides of the dollies, which occasionally have a useful shape for striking. They work.
I'm now trying to overcome my obsessive search for the nice old Martins at the flea markets. You'll occasionally find a few Martin hammers or similar in a box with a bunch of carpenter's hammers and priced accordingly. They are much more pleasant to work with, particularly if you's working for extended periods of time. No matter how many dollies you have, you never have exactly the right size and shape. I buy every one I see that's priced fairly. And with the bank account at .2 percent, what's to loose? As the collection grows, I don't use the cheap hammers very much but I still use the cheap dollies (they are harder to find at the fleas).
I'll warn you that the obsession only begins with hammers and dollies, though. Try to look away from chisels, punches, pliers, clamps. grips, anvils, steel tables..... while at the markets.
Tom