Garf01

Hello All, I wondered if any of you kind people can help me? I know absolutley nothing about cars, apart from appreciating the beauty and enjoying driving them! That being said I am currently looking into buying a 6 series BMW, I would like the convertable, as it's always been a dream of mine to own one. I only have a maximum of £12,000 to spend. As a generall rule of thumb, it appears I will get an 04, 05 or even an 06 (if I'm lucky) with ranging milage, obviously the lower the better. But assuming as long as it has full service history, it will last longer and hopefully less problems? Whilst looking I have picked up on the fact there may be an issue with Gear Box Sumps, is this the case, or am I panicking a bit? I would be extremely grateful for advise on this and on any other tips when buying, what to look for, & what questions to ask? Thank you, have a great weekend.