Hardtails are definitly the more lucitve Z50. Although I have never really followed the K0 values, they don't have lights so they don't attract the buyers as easily as K1-K2's would in my opinion. But it would make it easier far as resto goes, just remember that correct nuts, bolts, grips, levers, etc... make a BIG difference in value of these. Even if you are just doing a good clean up, don't go tot he hardware store and replace crusty hardware, clean it, polish it, have zinc coated... Also the hardtail was made for a few years. they're are ALOT of differences and keeping it true to the model it is can be important. If your bike is pretty complete already, it should not be to difficult.
I have a had a few z50 hardtails (K2's mainly) in the past and I currently have an original K1 thats needing a going through, I have always made money off them and I have had alot people contact me wanting my current bike.
I've watched a few softails, sit idle for sale and prices get lowered. They are a real nice riding little bike, reliable, and tough to. But they just dont have the following. they probably will though, as the hardtails increasingly go out of reach people will start saying "hey that K3 isn't so bad and is half the price" :thumbsup:
Good luck, and have fun