If you want a car that will eat motorway miles, that has a lovely and well crafted interior and that will run to Scotland and back from the midlands on a tank of fuel you can't get much better than an Audi A4 diesel of some flavour. If you want a car that is fun to drive, that has nice steering and good balance, that has RWD and is also a nice place to be with a good range of lumps you won't get better than an E46 3 series. I have no idea if you will get either for the money you wish to spend.
The A4's however are really not great driver’s cars. I have a new S-Line 2.0L Diesel on the drive which I bought for the wife and it’s a lovely car for the Scotland trip outlined above and has a simply beautifully built and laid out cabin. Having said this though it’s a boring car to drive, is very badly damped (sports suspension and big wheels are a recipe for this in Audi speak), has over assisted brakes and a steering wheel that communicates like Tommy (as in the def dumb and blind kid). It will sit on a motorway all day, is relaxing and comfy on a nice smooth motorway and now run in pulls more than well enough for something with no more than 140bhp.
Not on the same page as a 3 series however when it comes to driver enjoyment and if this is important to you buy the 3 series as the A4 will only disappoint in this area.