A year and a half ago, I bought a 2009 with 46k miles from my local BWM dealer.
It had no service history, the dealer bought it at auction. Cost me $5600 US + TTL.
Never owned a BMW before, took a chance based on the dealers reputation.
I've put 5K miles on it, no issues except a TPMS battery went out a week after I bought the bike.
The dealer replaced both TPMS sensors for $200, which was a signicant discount.
The air filter and alternator belt were new (changed by dealer).
Fuel strip still works.
I changed all fluids except for the clutch mineral oil.
Changed spark plugs and fuel filter. The latter was unnecesary - the old fuel filter was clean inside.
Checked valve clearance (all good). Did a throttle body sync and IAC cal using Motoscan.
Pulled the drive shaft, removed some minor surface rust and lubed splines.
Found a deal on here for a set of OEM shocks from a 2013 R with only 5K miles on them.
They made a not large, but noticeable, difference in the handling, so I suspect the original shocks were a bit worn out.
I found rust under the gas cap, cleaned that up and painted it, added the rubber ring.
Going forward, my main concern would be if the clutch wears out or has issues.
I would change it myself, but it would probably take me a month of Sundays to do that much work.
Beyond that, if there is a major failure where parts costs run into the thousands of dollars.
I don't think that's too likely but would probably get rid of the bike at that point.
Never ridden a camhead, from what I've read, it may be a bit smoother, the low-speed throttle response is probably better due to newer EFI tech.
My hexhead can be a bit tedious to ride in stop and go traffic and it has some vibration at idle and certain RPMS, but nothing terrible.
I've been tempted to get a low mileage camhead but I've kind of bonded with this hexhead.
If I ever replace this 09, I will probably jumpt to an LC bike.
I don't like the styling as much but they have been growing on me.
Functionally, I'm sure they are superior in many ways.
Have fun with your 06 if you end up getting it!