A little late to the party, just got back from Belize. Agree with Eric, looks to be a Z1 with a later model LTD seat [very comfy!].
I'm sure you know, Z1's are a very collectable bike and the value reflects that - when in stock configuration. I've seen the early ones go for up to $20K.
That bike looks to be very clean but the modifications would be a deduct if it is a true Z1 [only about 85,000 made]. Do you know the VIN's?
$7500 does not seem like a price if it is as clean as it looks, runs great and has paperwork.
As far as value, just like any other vintage bike, it is only worth what someone is willing to pay.
Go check it out, looks like he has some other nice toys in the garage!