I would say you want to try and get one with an upgraded stereo, like the HK, but that is personal preference. In all honesty, I had the standard speaker set-up in my e91 and I got on fine with it. It wasn't groundbreaking, but it did the job. I have HK in my F11, and I really do like it lots.
Pro nav is a must though. Gives you the bigger screen for a start, which just makes the interior of the 5. But will give you the HDD for storing music to. Unsure if it gives you the USB in the centre console, but don't buy one that doesn't have this. My e91 didn't and I hated that about it. And it isn't a cheap retrofit. I think Pro nav gives you audio streaming on pre-LCI, but it definitely doesn't on the LCI, but I believe it can be coded in.
520d would be a good shout. I didn't even drive one when looking, as it was the 6 cylinder I went for (I generally go through the numbers BEFORE a test drive, because if the numbers don't work, I have no interest in driving the car. After doing the numbers on the 520d and the 530d it was a no brainer to go for the bigger engine. But that was new. Probably not so much on the second hand market as you will no doubt get a better spec 520d for your money.
Sports auto is a must. You cannot get it on the 520d post LCI, but I believe you could pre-LCI, which is right in your budget. As Fox says, the manual isn't the best option on a 5 series, and the non-SA won't be anywhere near as nice. So might as well keep that one high on the priority list.
I would always want the SA, Pro Nav, xenons and M-Sport as minimum spec. But the earliest 5's didn't come in M-Sport, but had an extra package that gives most of the M-Sport spec. This is what Fox has. And is worth waiting for if they are not immediately available. My car has the M-Sport Plus but I would guess that's a relatively new option?