Ok, 400k is a relatively high asking price, but about par. Trade on the vehicle from BMW is about R200k, so see if you can get the price down a bit. Do some research. I'm seriously not joking about that.
Things to watch out for, as Crash said, they have bearing issues, but this was pretty much sorted out at the beginning of 2007, so less likely to happen on 2007 and up models.
The SMG gearbox is fine, if you learn how to drive it properly. NEVER use the auto option, it's properly shyte. I have mine in setting 5 even when not in M mode, and although a certain Splintheter on this forum constantly teases me about it, I love the mechanical feeling of the shift with the paddles. The other thing to keep an eye on is the actuators. There are 4 specific actuators that give issues. These are idle control actuators and throttle actuators. If they go, you're looking at R14k an actuator (last time I checked).
High pressure fuel pump in the tank is also prone to issues, that and the take up pump, repairs will cost R19k. I know, this happened to mine.
They're risky cars to be driving out of MP. They are expensive, thirsty (don't expect to average 15l/100kms, expect between 19 and 21) and you WILL fill it up every 250kms, especially town driving. Half a tank will last a day and a half, if you're lucky.
Most of the niggles were sorted out in 2007, but you never know. It was a R1.4mil car brand new, and there's a reason for that. Aftermarket mechanical warranties will cover you to a certain point, if you take one, you'll need the premium one that doesn't have restrictions. A new motor on one of these cars is in excess of R200k. That's already half your asking price.
Tyres, you will go through tyres very quickly. I had a set of brand new Michelin Pilot Sports on the car at the beginning of the year, my back ones need replacing after less than 20,000kms. If you go with the BMW standard tyre option, between R4200 and R5000 a tyre. I'm going to replace mine with Toyo Proxes T1 Sports.
But, if you're prepared to live with all of the above, it is the most epic car I have ever owned in my life. It is addictive. It brings out the hooligan in you. It's intoxicating, it's the most fun you can have with your clothes on. I love mine, to such an extent I bought a 330D to make sure mine keeps on going for a long long time. Look after it, and it will look after you. They're amazing cars. The engine is a masterpiece,the last and only naturally aspirated V10 M EVER. Find out from BMW about MP extension, if you can get it on till 2016, rather go that route. They MIGHT be able do it, investigate this route.
And your resale value is literally nothing. So don't think you'll buy it at R400k, and sell a profit, or get settlement if you decide to trade, trust me, you just won't. I have a friend who, for shits and giggles, got a quote from BMW for a trade in on a 2008 model, with 79,000km's on the clock, and BMW offered him R180k. Just saying.
PM me if you want any other info. There's a few M6 owners on the site who'll all shed different light on their personal experiences with the car. As for me, viva V10, viva!:rollsmile: