Mauro, an actual "clutch kit" was never offered by/from Mercedes Proper. Everything was always ordered piece by piece. Granted, some aftermarket manufacturers and retailers do offer a kit. However, everything you need is still available from Mercedes and aftermarket companies combined. I guess it comes down to asking yourself if you want to pay Mercedes prices or not.
Given that the transmission has to be removed to replace the clutch, you definitely want to renew everything that you can while the transmission is out. This would include the clutch friction disc, pressure plate, release bearing, clutch release fork and retaining springs, ball pivoting pin for release fork, pilot bearing for input shaft, and sometimes even the bearing housing that the release bearing slides on has to be replaced, as well. If the transmission is leaking this would also be a good time to reseal the transmission since it'll be out of the car and easily accessible.
The clutch parts used for the ZF 5 speed shouldn't be special or unique in any way. The ZF 5 speed and regular 4 speed manual transmissions are supposed to use the same clutch parts. (at least that is the indication the book and EPC gives) Do take Benz. Dr.'s advice into consideration regarding the release bearing. You (or the person doing the work) will have to determine which type of release bearing your transmission currently has. It's not something you can determine until after the transmission has been removed from the car.
And don't forget about your flywheel friction surface. Remove the flywheel and either have it resurfaced or buy a new one of it's damaged or worn past the maximum wear limit. Still available from Mercedes, part number 108-030-06-05. Currently lists for 712 Euros.
Currently, Mercedes has quit selling the pressure plate, and they have no plans to offer them again, so we're at the mercy of aftermarket sources. Oddly, Mercedes still offers the clutch disc. Anyway, there is a kit available here in the U.S. made by Sachs.
https://www.rockauto.com/en/emailpagetofriend.php?nurl=%2Fen%2Fcatalog%2Fmercedes-benz%2C1967%2C250sl%2C2.5l%2Bl6%2C1434935%2Ctransmission-manual%2Cclutch%2Bkit%2C1993. It nay cost a little more to ship it to Italy, but at least a kit is available. Alternatively, here is another clutch kit available in England. Though, use caution. I have no idea who makes the kit or if this place is reliable or not.
https://www.theslshop.com/parts-shop/mercedes-benz-w113-sl-pagoda-parts/drive-train/clutch/mercedes-benz-clutch-kit-0152503101.html And here is a place that sells just the pressure plate. ALVADI
https://alvadi.ee/en/spare-parts/mercedes-benz/pagoda-w113-1963/clutch-pressure-plate/21652-mercedes-benz-pagode-w113-250-sl-113-043-110kw-petrol-1966-1968Bear in mind that the release bearing included in the above kits may or may not be the one your car has. If the one in the kit isn't the one you require then you can buy that separately.