this is based on what ive researched previously in preparation of getting a brera, but by changing the headunit to an aftermarket one, youll lose the info on the instrument cluster (not a big deal), and youll lose the 'blue&me' functions (although i believe there are some work arounds, but not sure how successful they are). from what ive read, most people who leave the blue&me connected after fitting an aftermarket headunit do so to avoid the error on the dash, although this can be fixed using an option in multiecuscan.
bluetooth and voice control are built into most modern aftermarket h/u's, bluetooth especially.
the steering wheels are dependant on the replacement headunit. most support steering wheel controls, but check before buying. youll likely need to buy an adapter from the likes of connects2, although ive seen some that dont require one.
personally, i plan on ripping out the stock unit, and replacing with a parrot asteroid smart android double din unit.
steering wheel controls work, has gps, bluetooth, voice control for music search and phonebook, decent sized screen with a resolution that makes the oem sat nav look like its 20 years old, the ability to download maps from the market inc multiple sat nav apps such as tomtom/wave/igo/navfree, video inputs for rear cam/tv etc etc.
only problem is that once you price up the unit, the mounting adapter, the steering control adapter, and the possible cost for a multiecuscan licence, it becomes quite costly.
but then look at it