I can confirm that 16x6.5 is a great fit. Have test fitted a set and they are a good size that looks fine on the car...
Rims I use are orig. fiat coupe, but they require a set of spacers
Otherwise you should look for rims that will fit the fiat barchetta (approx. ET 25).
If you read some of the old posts,there's some pointers as to what will fit the murena.
I don't think 16" wheels should cause any trouble, as long as you can find them in ET:28 and pcd :4x98mm possibly 4x100 will fit if you use wobble bolts.