Look through galleries of cafe bikes for ideas of style and also note what motors are used. If you want to hit the highway and keep up, especially two up, you need at least a 550. Of course some of that equation involves your weight too. I know Hondas best, because I have a collection with every four except the 500 (one on the way), a CBX and a few other bikes.
I don't believe for a moment that the fours make lousy cafe bikes. Just look at their use in vintage racing. If they were such slugs, too wide or heavy, they would be trampled on the track.
If you are a light weight rider, the CB400F is a great bike, lots of pep and a 6 speed trans. It's really a shame that Honda didn't put six speeds in more of their fours. The CB550F or K is really quite surprising for pep and handling and the CB650 up to 80 was a really cool bike too. Mine is a great little rat for short haul riding. In 81 the CB650 got CV carbs and the suspension geometry was made more cruiser-like. On the SOHCs, put a 650 cam in a 550 to get better mid-range. The 400F motor bolts right into a 350F, the 650 likewise fits a 550 frame.
So, there's several considerations - any of the older 4s with smooth bore carbs are also SOHCs which a lot of guys prefer for simplified maintenance. The SOHC 550F or K and the 750F or K, have single left side disk up front but the fork legs & hub are set up to accept a right side disk, giving you duals. The smooth bore carbs give snappier performance because you're not waiting for vacuum to lift the slides. All of these older bikes use ball bearings in the steering head. Search for All Balls Racing and get a kit of tapered bearings. This alone will improve handling a lot. Also usually the swingarm bushings on older bikes need to be replaced. Similarly, do check the wheels and wheel bearings, and do a thorough brake refit. On any 30 plus year old bike, do the brakes, chuck old hydraulic lines and replace with SS lines. Use DOT5 fluid and then a spill won't wreck your paint. That applies even to the older DOHCs. None of the single disk version DOHCs are easily converted to dual disk. It's cheaper and easier to buy the forks, brakes and wheel from a dual equipped DOHC model but you may also need the triples. They are not all the same width.
Honda started using CV carbs about 79 on some bikes and by 81 I think all models had them. Also, all of their older DOHCs (750, 900, 1000) used CV carbs that work best with the original airbox because it's part of an engineered system.
Watch out if you're wanting to run a wider rear tire, because the swingarm isn't your only problem. You're width limit will be determined by the distance the rear sprocket sits from center. Otherwise if you run spacers or a wider hub from another bike, it won't line up with the motor.
The drawback on the DOHC is minor but made into a major issue by some people - the cam adjustment is "bucket and shim". Join DOHC Customs (CB750 dot com is NOT only for 750s) and you'll access all sorts of how-to guides and helpful guys. There's lots of DOHC 750Ks around for a reasonable price and the motors are strong. If you want you can get a big bore kit for them, taking them up to 836cc. Throw in a set of 900 cams (900C & F use the same cam) to avoid the typical Honda mid range blues. Hope that helps.
Instead of trying to decide on which bike to buy, get one of each like I did 8)
