I would go with a 650, since it does not have the secondary air system. The 645 will have the SA system, & the head air ports for the SA system will eventually clog. Once this happens, it will not pass inspection. Any e63/E64 will eventually have the valve seals fail, & the cost is not cheap. The price now is somewhere around 2500.00 to change the valve seals. Our cars valve cover & upper timing cover gaskets will fail in the 40 to 50k range after the repair. There is not anything such as a good buy on a 645 or 650, as something is going to fail at some point. If you can do your own repairs, that will drop the repair cost when needed. Our cars are like women, they love to have money spent on them.
Not trying to scare you away, just make sure you are prepared of what the cost of repairs on a 90 to 100k car will cost!
I'd go for the 650 if you find a good one and you can swing it.
I'd think at this point it's preferable to get one that already has had the oil seals replaced, especially if it was recent and done properly. Ironic I should say that as mine is a car that is still on its original seals and I know replacement time has to be fast approaching. Then again I've been thinking this for the more than 11 years I've owned it so...
Don't worry about the secondary air system, we don't have to have it working in the UK, if it fails, unplug the power line from the pump, cover the connections, tape them up and forget it, we don't need it in order to pass the MOT and it doesn't harm anything.
It's rare but you do sometimes see very early 06 650's for sale (needs to be before March/April I think) and still be in the 645 tax bracket, bear in mind though the infotainment system will be older than your used to, not sure if that will bother you too much but there are ways to change that.
645 and 650 running costs are pretty much the same except for the above mentioned tax.
2005 Tax time is a v rare find for 650 so I'm mentally prepared for higher bracket. The sound system is Logic 7 but not of much use in its current form. I will probably get an aux Bluetooth receiver to work out.
Oil impacts 99% of e64s the way i read. Im probably also concerned about additional roof issues in convertible reading up the forums. My 2013 somehow never one falter on the roof.