White "smoke" out the tail pipe when cold is just water vapor and nothing to be concerned about.
Tach drive leak is just a replacement oil seal.
Rattle at idle might simply be a carb vacuum sync issue. Hard to diagnose a noise over a text medium, though.
The turn sig relay is voltage sensitive, and the voltage is lowest during idle. I routinely use the flash rate to tell me when the battery voltage is getting low.
If they stop, it could be as simple as the wrong bulbs were use in the signal, Also a weak battery could be indicated. You will need a voltmeter to further diagnose, and if you need it, maybe a new flasher unit. I certainly wouldn't let it prevent a buy.
Sounds like a good bike. Don't be surprised if he sells it to someone else. But, you might get lucky.