Hi,
I had a 2006 IS250 se-l which I bought for £5k and funnily enough I also owned an E46 325ci. The abs module in that bmw drove me mad.
Over 2 years of ownership the is250 was uber reliable. All electrics, engine and gearbox never gave me any issues.
For the first year I made short 10mins journeys and after a change in job went to commuting for an hour. It did both really well. The is250 is such a relaxed motorway cruiser and I still miss the smooth gearbox and engine. Perfect combo. I just made sure to service it on time and at 60k miles it should have had a spark plugs change as part of the full service.
Only negative was that the 17inch alloys corroded badly. Not sure if this was an issue on the 18inch alloys.
No idea on aftermarket headunits however if you go for an se-l it will likely come with the upgraded mark levinson sound system which I found very good but I'm not an audiophile.
As your looking for a car from 2007 onwards, it should have the upgraded satnav/mark levinson that allows you to stream music over Bluetooth from your phone. Mine being a 2006 meant I had to use an aux cable to my phone.
One last thing, if you do get the IS250 always make sure the fuel filler cap is on properly after refuelling. It should click close. If you don't it will throw an error code on the dash. Easy fix of course (switch off engine and tighten the cap) but can be alarming when your used to supreme reliability.
Hope this helps