On the less-cost (but adequate) end of things, the CruzinImage piston kits work well. But, like BryanJ mentions: you'd best pull the head and measure the bores before picking pistons. Don't just try to re-ring: this only works on the 750 if the engine has less than 5000 miles on the present rings, and then it only lasts about 10k-15k miles. It will not restore the performance that was lost to the warping of the cylinders as it cured the sleeves, either: to regain that (and more) you must bore it at least a step-size if the bores are virgin size.
None of the current crop of gasket kits are complete. The NE brand has the most parts, but (in my experience) the jury is out over how long these last because they only appeared about 6 years ago. I have used Vesrah for many years with good results, but they, too, are missing the head seals (PartsNmore has the correct ones) and some of the O-rings (you can contact me for those). You can also get a decent shaft seal kit from CB750Supply and PartsNmore, among others out there. Most of these today are being made by the same vendor and are packed with differing names on them, but are the same parts.
If you're embarking into the 750 engine, maybe think about my book: it goes step-by-step thru the whole bike and gives all the pertinent numbers and details. It also brings some $ support back to our forum!
