Just to add to what Fox has been advising, if you are interested in running a warranty on the car, which on a 3 series is relatively cheap, try to aim for less than 60k on the mileage.
After 60k the warranty cost roughly doubles, but if you take it out before 60k, and pay monthly, it continues to roll on for as long as you pay it at the reduced cost, pretty much regardless of mileage.
And in the unlikely event you manage to find a BMW Approved Used in budget, it'll have 1 year warranty (2 if you can negotiate it), so it will be the mileage from the point at which that warranty runs out. So if you buy one on 40k, and do 15k per year, and you get 2 years warranty, you will be over that threshold. So be aware if warranty is a thing you fancy.
As an example of warranty costs, I think I was around £280 a year for the warranty on my 335d, which was sub 60k, on a 2010 LCI. But I think the cost is the same per series, regardless of if you go 318d or 335i.
As for differences between Business and Pro nav, the easiest way to tell is screen size. Pro nav has the widescreen, business doesn't. Different version can be seen by the iDrive controller. 2 buttons on the older CCC, and 8 (I think) on the newer CiC (and NBT, but that'll be on F30's out of your budget).