I'm no audiophile, but I think they have certainly improved the speakers a great deal when going from the Laguna I to the II. The quality of the head unit seems to be a lot better too, though definitely nothing to write home about. Crank it up, and it'll distort to hell well before it makes your ears bleed.
If you're serious about an upgrade, then a head unit swap is probably the way forward - then see if the speakers can cope with it. The good news is that it's a standard DIN-E unit with ISO wiring, so it should be pretty much 'plug & play.'
Some stereo manufacturers have optional interfaces to allow the remote and external display to work. Shop around and see what's available, but ask yourself if these really are essential features before paying too much extra for them. I swapped the Blaupunkt in my first (M reg) Laguna for a Kenwood radio/cassette/10 disc changer system that I already had, and didn't miss the remote control or display one bit.
I looked into it a while ago, and you can buy interfaces to connect certain brands of CD changer to the standard head unit. Some of the interfaces work with 10 or 12 disc changers, but the head unit will still only let you play the first 6 discs. Second-hand proper Renault/Alpine 6 disc changers crop up occasionally on
too, though I doubt they come with the cable, and I don't think it's fitted as standard on cars that come without the changer.