Simpleman,
Your going to love the Z4 roadster not matter the motor size
I went with a fully loaded 03 3.0 which was hard to finds 4 years ago and it took a while to find one. I love it and drive it 99% of the time with the top down.
For me it was decision of M or 3.0 and since I wanted a top down cruiser that would get great fuel economy I went for the 3.0. But I will try a M out one of these days
Personally I love the looks with 18" rims so that's the only thing I'd change on your 2.5.
Aftermarket HID are cheap and easy to install but I don't think you'll get the focus of the factory ones. I did just install a set of H11 HID's in my driving lights to match the factory Xenon an am happy with them.
Post those Pics when you get it