Hi Dave,
If you know what you're doing when buying cars then why not go private? More bang for your buck, with so many dodgy dealers and even main dealers just plain lying through their teeth on 2nd hand cars you don't always get what you think. You can buy an independent warranty if you want one, and a 3 litre E89 shouldn't have any major faults. Unless new I go private every time, be careful, choose the owner on location and attitude and you will pick a peach up for a lot less than from the middleman. There are genuine good sellers out there if you know what you're doing IMHO.
Best of luck.