I've got the old laguna and cheated. I used an fm modulator into the aerial socket of the renault hu.

For my the mark 1 laguna you can get a 'connector' that converts the renault stalk controls into sony (its not sony currently). This means that you'll lose the head up display part for the radio station etc.

https://www.edworthys.co.uk/acatalog/Fitting_Renault.html

Renault Smart Lead (PC9-112) is the one you want I believe.
I'm just about to move back to the UK and was looking at getting another laguna (mark 2 obviously now). Mine won't have the spare slot as thats where the satnav is, however the little headup display is replaced with a colour lcd screen.

Either way, the factory installed hu was coming out and I was going to replace it with my empeg and an amp. Might be worth speaking to your local renault dealer about this.

The other car I'm looking at, is the new Mazda RX8. But that has some purpose built Bose crap installed so that won't be an easy task.