I've had my E85M since late Nov. My old man had an E46 M3 cab, so we were very aware of how expensive it is to fix an S54 that's been abused. My old man spent far too much cash trying to sort his M3 out in the 7 years he had it.
With the budget I had been given to get myself a car, I was not able to stretch it enough for the Z4M Coupe (E86M) , but I was able to find the car I now own locally, which is pretty rare for Oxford.
Unless you plan on turning your Z4M into a track day weapon, I don't think you're going to really notice that the roadster is not as stiff as the coupe. I asked this same question and I was given this answer and through owning one, I know this to be true. The chassis is really impressive with the roadster and for B road hooning, I've never found the roadster to suffer the same issues a E46 M3 Cab does when compared to the coupe.
The steering rack in the roadster is different to the coupe, I know it's the same rack as the E46 M3 in the roadster and it actually suits it pretty well. The weight and feedback has been something I really liked with mine and the thicker rim, does make it much nicer to interact with. When I am bombing around the various A and B roads in Oxfordshire, I've found the car to handle pretty well in the bends. The one negative I've found, is the rear end is upset pretty easily on roads that have a lot of sudden dips/humps. Personally I plan to put a slightly softer set of rear springs on my car and I have been told by senior members on the Z4 forums, that this is a common enough mod and actually makes the rear feel way more planted.
With mine, I have found that until the gearbox is warm, 2nd gear is a bit tricky to get use to and it seems to be a quirk found with the M's 6 speed box. I've managed to clock up about 2500 miles in my M since I was bought it. For the last 15+ years, I would average 3000 miles a year, even when I was very active in the JDM scene with my Impreza P1. I have nothing but positive things to say about how just utterly amazing my M roadster feels and drives.
Even my old man has been blown away at how tight and well made the car feels. BMW and the M division really managed to turn the Z4 into something very special. I don't think you will be disappointed if you got a Z4MR. With the weather being much improved, I have been really enjoying the roof down experience, so much so that I am not lusting over the Coupe's as much as I had been.
I've been very lucky that my Z4M was actually a 2 owner car and it had been well looked after, so much so it was pampered. Typically with the roadsters, it is pretty common for them to have pretty much a new owner ever 12-24 months, so don't be put off if it's getting close to double digits with previous owners. It seems that some people just find them a bit much to live with, as they are sprung pretty firm and have a rawness to them. Service history, like with all M cars is so important, which is what ended up causing my old man to have so many issues with his E46 M3 Cab.
I recently watched a video where it was pointed out that in a climate like we get in the UK, the recommended oil might be too thick, as it can be difficult for it to get upto a good enough temp for it's thickness to have reduced well enough to protect elements of the engine. I am not very knowledgeable about this type of thing, but it's something I am going to chat with my local BMW specialist when I take it in for it's next service. Because of my history with JDM cars, I am use to having an oil change every 4000 miles, which is way under the service interval my Z4M's computer indicates. I think the S54 would actually suit having more frequent oil changes, so I plant to get it done every 75% of the indicated interval as a preventative measure.
One thing I have been pleased with, is how good the MPG can be with the M. If I've just been driving around and not caining it, I will get about 26'ish MPG. On some of the long journeys I've done, getting the MPG into the mid 30's is pretty easy and that's without really trying. When you really get on it, I've been getting 20-22 MPG, which considering how rapid the car can be, is not too bad. My E30 Alpina use to give me 18 MPG most of the time lol!
I have been lucky to have owned/driven a fair few quick cars and I never expected to fall totally in love with my Z4MR as quickly as I have. While I utterly adore the Z4M coupe, I am now finding out that I'm not really too bothered about having to get a roadster instead. It's just a very good car, that is able to make every journey enjoyable.
If you have any questions about the day to day living with my Z4M I am happy to help. The more technical stuff, I am not really that well versed and could easily give you information that might not be correct. Something that might be useful for you to know, that even with me being over 6.4" and 20 stone, with lower spinal damage, the sports seats and space inside my roadster is actually pretty good. I do have to lean the seat back a bit more than I was use to, so I could see the guages easily and be comfortable. With this slightly more leaned back seating setup, I have been able to drive for much longer with my back problems, than I have been in any other cars for years. I was worried the Roadster would not be big enough for someone my size, yet it does not seem to be that close to the limit with me.
If you're close to the Oxford area, I am happy to meet up with you and let you try my M roadster out for an afternoon, to see if the M roadster would work out for you over spending another £6-£8k more for a coupe.
