I bought a 35is a couple of months ago after searching around since last summer.
Please do not buy that car. It's been on and off Autotrader, Ebay and the likes since July last year. I know because I recognise the pictures and the plate - I enquired about it.
It's been in to BMW several times after having a front end crash repaired. New bumper, bonnet and wings fitted but lingering radiator and turbo issues. It's a mine field, there are much better examples around.
Like Fox says, for £18k you'll be better off getting a decent 35i that has been looked after. Navigation is a must, the interior looks nicer for it and iDrive is an awesome system. Your budget should get you a nice 2010 35i with less than 50k on it, most of the toys.
326Ms are the best wheel, Advanced Loudspeaker is worth it over standard (but still not a "great" stereo) and I think parking sensors are required. Fairly sure parking sensors are standard on a 35i. For me the DCT box is the one to go for, but there is the understandable argument about a sports car needing a manual.
If you go and look at one, check the wheels for cracks. The Style 296 are pretty bad for it and getting them fixed under warranty is a ball ache. 326M known to crack but much less likely. Also have a look at rear light clusters for condensation - dodgy seals on the early units can lead to failures, BMW warranty will replace free of charge assuming the lights aren't damaged.
I've found prices for the 35i/35is fall £1-2k once summer is over, BMW main dealers will charge £1-2k more than private (obviously).