Not specific to Italy, but I have found paying extra for such "features" at Sixt to be rather a waste than an asset that gets one a nicely loaded or upgraded car. Even GPS / Navigation System (build in) is often standard on many cars these days, especially for Mid- / Large sized vehicles. Diesel I would only take if you really want it / need it and will drive a lot (lower consumption in theory, but diesel prices are not low any longer).

The only extra I find important to confirm if you really need it (e.g. in winter season) would be all-wheel drive, which I have seen as an option for some higher car categories like F (3-series, A4 Avant etc.) or L (5-series, A6 Avant etc.).