Geeto sounds like you already got bent.
I am new to this and I am learning my way through. My statement about my budget was "tongue and cheek" and you sound like a sensitive person. I cannot afford an upfront cost of $4,000 at the moment. I should be able to scratch together $1,500 -$2,000 in a month or so. Is that enough of budgetary info for you Geeto?
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Hardly a sensitive person...but being stupid or coy about things only hurts you not me. I'm not the one seeking info on the internet I'm the guy who has owned dozens of the bikes you are looking for.
Regarding the KZ900/1000 you mentioned before: I don't think you are going to find a 1973-75 Z1 for your price range without a lot of issues to go with it. You might be able to find a 76 kz900 or a 77+ kz1000 in that range but by then the LTD models were out. The early ones aren't too bad, but later LTDs have too many different parts for you to convert them to anything other than the factory chopper/crusier without great expense. Try to get a 77 kz1000 if you can, they have rear disc brakes. Also consider the 1981 GPZ 1100 since that is the bike that Eddie Lawson made famous.
Considering you are new to this and with your budget (which to me is a little unrealistic - for a good clean bike with no issues I would budget $2500) I woud stay away from bad ideas. That 550 shop bike is a bad idea. Anything that isn't running, with a clean title, currently registered, and being actively ridden is a bad idea. It is more expensive to fix a cheap bike than it is to buy a nice one in the first place. These are old bikes, you are not going to run out of tasks or things to work on if you are using it as intended.
Regarding a CB750 - with your budget focus on a 77-78 K bike as they seem to be the best "value". You get all the good of a cb750 for less money than a 69-76. If you are going to cut it up anyway (and it seems like you might) then it doesn't matter that it doesn't look like a 69-76K. Someone alreafdy mentioned the big valve head valve guide issue.
Keep in mind, the purchase price of the bike is only the first bit of the expenditure. You still need to budget for insurance, proper riding gear (which is not a bell 500, a t shirt, and jeans), and the basic service items to give the bike a tune up once you have purchased it. Sock away another $500-$700 for it.