We have a guy in the club that does a great job of going thru the card. Steve Sefsick.
When he rebuilds he mods them so the engine can't hydrolock and rejetts for more power.
I think the cost is around $300 and you have to remove and send the carbs to him.
Local shops may do a carb rebuild for you for about the same $$, but it won't have the same improvements he offers.
If it's hydrolocked, you don't replace the piston. You replace a bent rod and a piston.
Pretty expensive...
The odds are that the bike your looking at, isn't hydrolocked.
If you can get the owner to let you perform a hydrolock check, probably worth the $$.
We can link you to a video that explains how to do the check.
As your not used to working on the bike, it can be done, but most owners wouldn't let you do the check...
If it was hydrolocked the cost of a reapair is too much to buy the bike...
Ride safe, Ted
PS: I have no idea where your located, but it seems you have $2000 to get a running Connie.
Personally, I would opt to buy a running bike...
Here is one on the COG Forum for $1800 with a lot of options on it.
https://tricities.craigslist.org/mpo/d/fs-2000-kawasaki-concours/6655666658.html