I bought an 84 318i coupe for Ԛ£250,body was shit,but it came with a MASSIVE history file,on tick over inside the car you couldn't tell whether or not the engine was running,it had 170k on the clock as well,it ran fantastic.
It was mechanically perfect dues to the money spent keeping it together.
This tells me service history is important.
You'd be better to buy a high miler with full history than a low miler with no history IMO.
MPG wise,mine used to last for ever on a fiver,so good infact it just wasn't an issue,but i would say I gor around 30-45 on average.
Rust wise,check battery tray,jacking points, front inner wheel arch at the bottom,under front windscreen,all wheel arches,front suspension turret areas,inside the compartments in the boot on the left and right and sills,moreso if you buy a car with kit,although difficult to see,check it as best you can,some people have had had nightmares under their kits,others haven't.
A cambelt change is a bonus too.
HTH a bit,good luck with your quest!
