Seems it's more of a budget issue than a car issue. Having owned a Westfield the OP will know what excitement they are trying to regain. I don't hear anyone worrying about the 3.0 resale values plummeting on here. Why not get a decent 3.0, give it a go? If it doesn't work out there is no massive losses. If any losses depending on mileage you put on it. If you get the M then don't maintain it properly, you will want to sell it. But it will be a hard sell.
Yes cost is an issue pockets are only so deep but always wanted an M3 since the early ones in LHD only so over 20 years ago but the price of the M3's has gone up too much, so the chance to have a Z4 come along and I took it don't regret it at all thought about going to a 3.0 an as someone said 80% of an M for 50% cost but as buying and selling car has always come with a loss or cost to change it would probably be better to change just the once.
To be clear I am looking at the lower cost end and it would be a second car and use it 6 months of the year on nicer days.
The maintenance I would be looking to do as much myself as possible having built a Westfield, two engines and part rebuilt a Landcover defender and have worked on CCGT power stations etc. and as a few people have said they would rarther an enthusiast do it than a main dealer. Took me 2 weeks, 2 hours a night to get the Duratec cam shims right as I had lapped the valves in after re profiling the valve angle and to avoid buying 16 new shim buckets I shortened the valve spindle to suit but you have to rebuild it every time and I aimed for all the same not just within tolerance just a bit of OCD