the kawi twins are for the most part UJMs. Reliable, a little slow, and the majority of them are crusiers (LTD models). One of the things I have always disliked about them is that they are not simple to do engine work on. There are no less than 4 internal engine chains that you have to worry about:
1) timing chain, normal maintenance just like any UJM (ubiquitious japanese motorcycle)
2) Starter chain, these go bad just shy of 20,000 miles. Depending on the model they will start to chew up the clutch pressure plate until catostrophic failure occurs. Nonoe of the parts are cheap BTW (some of the 80's ninjas suffer the same problem)
3) balance chain, does exactily what is says - there to balance out vibrations in the motor. When they go bad the bike shakes itself to bits. very rare that they go bad though.
4) primary chain, typically a hi-vlo chain that really never goes bad.
on the plus side they are cheap, and a nice hot rod project that would be fun on low cash would be to take the kz440 ltd 440cc motor and put it in an early (and much lighter) kz400 frame. the early kz400s make nice little cafe bikes so long as you know that if the motor grenades itself you pretty much throw it away (the chances of the motor grenading itself are pretty low).
Kawasaki made a kz750 four and a kz750 twin. The kz750 twin is an appauling bike. Half crusier, half muscle bike and it does neither well in any reguard. It is uncomfortable, bulky, and overall not a great handeling bike. They also suffer from electrical and carb gremlins to no end. I had a friend in New Orleans who owned one and it was constantly needing something (and it was a farily clean low mileage bike). Awful to ride. They don't really suffer from any design flaw or shoddy manufacturing, just the components they were made out of have not proved as durable as other bikes like cb750s and gs750s (and even the kz fours). Made from 1977-1980 (approx) it made 55 hp and weighed 480lbs dry (so about 510-515 wet). Even the mose anemic honda SOHC4 750 made more and weighed less.
The kz750 four is actually a decent bike but not very popular when new. Kawasaki had the kz1000 as it's top superbike during that era and the kz650 was the middleweight. The kz750 four was heavy, slow, and constantly had it's lunch eaten by cb750s and gs750 which were direct competition for the 1000 KZs. it made 74 hp and weighed 462 lbs dry (so about 500lbs wet). Made in standard form for 1980 only and LTD form from 1980-1981, the other mfgs already had 900s and 1000s in their stable that weighed the same and made way more hp. the detuned heavy kz1000 at the end of its life made close to 100hp (96 actually) and weighed 520lbs wet. I am not 100% certain on this but I think with some updates the kz750 four, in a new chassis, became the gpz750 and eventually the gpz750 turbo