i have a 2.5td if they are warm they take a few more turns for it to start so i wouldn't worry about that also remember there a big 6 cylinder engine so takes a fair bit to get them started. if you didn't let the car heat when it was cold (i.e the engine heat light to turn off in the dash before starting the car) it will get it hard to start as its diesel! the diesel omega don't actually give out much smoke at all unless you drop the car down a gear to get power. i wouldn't worry about the black on the exhaust its diesel it happens and will be all over your boot soon!!! that idling speed is right mine is the same. the way of knowing if the car is idling too low, the car engine will be under pressure and start chucking while idling.
i would check the sump to make sure it hasn't been re welded especially around the sump plug as the sump is very low.
if its an automatic id be cautious about buying it as the auto box isn't as good as the manual and give a bit of trouble i think
wouldn't worry about mileage cause they go for ever mine has 225000 miles and still running perfect and know they can get 350,000
good luck