Allow me to chime in, but since this is my first official post, let me give you my credentials: I've owned my 78 cb750 for less than a year, and I am going cafe style with the bike. I've also recently acquired at 77 parts bike (another K model). My riding goals seem similar to your's: cool looking (ie. cafe racer) commuter bike (ie. reliable) that's also fun to ride (ie. with balls). I buy most of my parts from ebay. I've got a pic of my cafe project on this thread:
http://forums.sohc4.net/index.php?topic=33548.msg482761#msg482761sohc4 experts, please correct me if I'm wrong on anything that follows.Now what you are willing to deal with determines the answer to your question (to quote other posts in this thread -"it's relative".) You probably shouldn't be looking for a 69 or 70, as those are considered highly collectible, and I respect the 750 too much to ever want to cut one of those up. Addtionally, the collector value on those would be much higher than the prettiest cafe racer you could ever build. K0 and K1 have sold for $5000-$15,000. Most cb750 cafe racers I see being sold only fetch $2000-2500 at the most unless they are made by Steve Carpenter. Parts for these bikes are similarly expensive if even able to be found.
72-76 would probably be the best choice. Lots of stuff available for those including carb kits, sprockets, spoke kits, exhausts, and body work with most of the aftermarket cafe seats made for the 750 will accomdate the hinges and locking mechanism on those bikes so your cafe seat will open/close/lock down just like the stock seat (77-78 hinges are on the other side). The stock tanks look cool too with those flip top gas caps. If you're into cafe racers, then you should know Carpy, and he almost exclusivley only messes with these year bikes. The K frames on these bikes are among the shortest of the 750 sohc series (77-78 and the F bikes will most likely have some frame tabs (turn signal/rear fender mounts) extending out from the bottom of your cafe seat -I had to chop mine off on my 78). The 77-78 K models were designed to cater to the touring market, so they got longer, more rigid (ie. heavier) frames and Honda designed the F models for the sport market (supposedly thinner walled tubing on the frame (ie. lighter). I think the F model frames might be longer also.
77-78 were probably the most reliable 750sohc engine (except for the late F model). Any refinements Honda could do the the sohc750 engine were done to the K7-K8 motors (3 piece oil rings, increased oil cooling, heavier duty 630 chain, etc.) Parts are a little harder to find, but since there were fewer of these bikes, the parts are in less demand (ie. cheap).
The F models seem to be the least desirable of the sohc750 bikes. In my area, you can typically find them for about $200 cheaper than the same year K model. The 1978 F3 had the most performance enhancements designed into it including the bigger valved but questionable valvetrain. Three disk brakes are cool, but from what I read, the single disc/drum brake set up already performed well. I have dual fronts discs on my bike, but I did a GL1000 conversion to also get the benfit of larger diameter forks and aluminum rimmed wheel. Also regarding the F3, the cafe look becomes hard to achieve with the the stock ComStar wheels.
