I can't believe we're on page 3 of this thread and no-one has mentioned the VIN lookup tool.
http://www.rubmw.ru/vincode/eng/
Punch in the last 7 digits of the VIN of any car you are looking at to see what the factory build spec was when it was new. This will list what type of HiFi it has etc etc. it may not say 'carver' specifically (mine has it but says "top HiFi" on the VIN lookup).
It's well worth finding a car with the carver system if you can, I think it sounds great and if you get a car with Nav, there is a full dsp equaliser screen that you can adjust all of the frequency responses to your taste.
There are threads on here where a sound engineer has sat in their z4 and tuned the frequencies to perfection using test equipment and they've published up the settings. I use a variation on those and I'm very satisfied.
As a comparison I've had Bose in a 911 turbo, harmon/kardon in my MINI and harmon/kardon in a range rover and I rate the z4 the best of them all. I have had an s1 Elise and s2 exige s and know how awful the radios are in the vx220s too.. Anything is better!
The 3.0 model comes with a lot as standard as I expect you're starting to see. Electric memory seats (heating was optional), sport button, rain and light sensors, headlamp wash (xenon optional), leather, climate control, metallic etc.
Notable options (if you don't already know) include; professional Nav, heated seats, HiFi upgrade (you know this), cruise control (cheap and easy retrofit), sports seats on later models, xenon lights, Bluetooth, wheel upgrades obviously, different leather on standard seats with extended leather options (this is rare but very nice - leather sun visors, leather all over the door cards not just the middle, windscreen surround etc - look for New England leather), parking sensors (pdc) etc etc.
Good luck in your search - plenty out there and you'll find a cracker.
Edited to say - the MFSW was always an option on early cars and seems to be reasonably rare. It can be retrofitted to the standard or sports (thicker) wheel for approx £200. It's on my list of things to do.