The problem with carb kits is that they are ferociously expensive. Were I to assemble a kit consisting of 39mm Keihin FCR MX carbs with choke and hot start, plus cables, filters, and flanges, I'd have to charge at least twelve hundred for the setup. Used FCRs and Mikuni -86 and -6 series flat-slides can be had for far less than new. It doesn't seem to matter what the new kits are sold for -- they always seem to have the same jets, needles, and all other metering bits.
I counsel the following: do not even consider going to a single carb setup. Period. It will turn your potential sport-bike into a toad. If you want to have low end torque and not be able to rev, buy an older single. Now then, for stock motors consider 38mm Mikunis. For full-liter with some performance enhancements, TM40-6 Mikunis -- or if you like to tinker, 39mm FCRs. If you can afford them, get your FCRs in the MX configuration. The enhancements make for a much more tractable ride. If you stroke your crank and/or run a bore big enough that the case spigot must be enlarged, or run over-size valves, or really wild cams, consider 42mm flat-slide Mikunis or 41mm FCR MXs. If going all out 45mm Mikuni pumpers or 43mm FCR MXs.
At this point, it becomes more cost-effective to run EFI with used RC51 throttle bodies (55mm each!). If you really like to tinker, this may well be the perfect solution for you.
Of course, giant carbs do little to enhance power without extensive porting.