The 4 will be out of budget but if the M2 is "just a step" above the 1 series M costs (say circa 45k new) then I think we're looking at used values after a year of mid 30s.
Furthermore, I expect to see sub £350 lease deals on the M2 as well if that is the kind of price.
Which is (at least for me ... ) a damn sight less than I pay right now and therefore ticks both the key boxes:
2. There's a USP to convince the other half it's a good idea!
The M4 on the other hand? If they are really talking 60s for one of those when the M5 is only 70 something then I really can't justify it. Plenty will buy them. But not I.
Maybe I'd look at a 2 year old one of those, who knows.
The absolute most I'd ever splurge on a car is 40 - with my current circumstances. One way or the other.
Which reminds me, I was looking at Baby Astons the other day which can be had for significantly less than that now!! Tempting until I looked up what the gearbox costs when (yes, when. It will happen) it does. Circa a third of the value of the car. Oh well. Also - they weigh so much that on a very good dry road they aren't much quicker off the line than the 335d anyway.. hah. (Flame suit on)