Hi Abangr
Sorry if this response is a bit late! a 2009 5 series has an aluminum front end structure (last of previous shape, new shape 2010 onwards), so look closely for any signs of impact damage, as these should only be repaired by BMW Aluminum specialists, damage can easily be hidden behind plastic bumpers etc. (Check gap lines carefully in the bonnet) I would also recommend that you check in the boot where the spare wheel lives (If there is one) most have run flats now. There is a control unit (BCM) in the bottom of the spare wheel holder which is not uncommon to find it in a pool of water due to the boot seal leaking. Don't stress about the mileage, as long as the engine sounds well. (520i may be a bit thirsty compared to 520D)
I have 15 years working for BMW's main competitor and the last 6 working for a large Swiss insurance company back in Ireland.
Please feel free to PM me if you need further advise, Cheffin