i dont think stealth cars will be, but im sure chrome line will be a DLC.
stealth doesnt seems to be one of those time exclusives.
i dont know about sounds but its more power then the normal ones, not to mention cheaper to overhaul and restore, lolz so cheap that it only cost 500cr to do an overhaul LOLz
In my opinion they do have a different sound.. The McLaren F1 GTR has a very bright sound, but the SLS AMG has a very dark sound. At least from my experience with the normal to stealth model driving.