Before you go dropping it off anywhere to replace the valve seals, get someone with more experience to witness the phenomenon. An occasional puff of smoke randomly on 1 in 10 startups, which isn't even confirmed to be either oil or coolant, is definitely not a reason to start tearing a motor apart.
How sure are you that its not even just the normal water vapor formed when starting a cold engine? Under what weather conditions do you see it? Does the cloud dissipate fairly quickly or does it hang around for greater than say 30 seconds? What does it smell like, can you compare it to the exhaust smell of another vehicle?
For reference, to replace the valve seals is going to be alot of work. Two cylinder heads, 16 valve seals per head. I do not know if there is a tool to replace the seals with the heads still bolted to the block. Some DOHC's you can do it, some you can't. Either way, I'd guess around 3K if you took it to a dealer.
rich