You need to wait until a Ulysse owner of that generation logs on - I have no idea what the stereo system is like! If it is a standard DIN fitting, just replace that. But no idea how that would integrate with steering wheel buttons.
I was going to replace the built in radio in the Espace and Pioneer do all sorts of interface kits to allow connections to existing multichangers and steering column controls. Try their web site.
Simon, I would speak to a specific auto audio shop to find out your options, such as interfacing kits for the controls, if they are too expensive, use what they told you to find it on the internet for example.
I have the same model year and thought the supplied readio cassetee was rather naff.
I upgraded to an Alpine head unit with 6CD mutliplay. The CD is located under the drivers seat.
I also bought the kit to ensure the steering wheel controls worked. Total cost about £340 and fitted myself (longest job was running the CD cable from the head unit to the drivers seat)