Hiya! Ideally offset should be from 28-to max 40 but the higher you go above 32 you will need spacers to compensate. The standard wheels are 32 but still use a 5mm spacer at the rear so any wheel from 28 will fit. Now the new Fiat 500 has been reproduced there is some nice choices to be had with the original 4x98 stud but you can use a 4x100 withe the choice of wobbly bolts or the use of hub adapters depending on offset. (38 upwards)
One word of warning though! 17" wheels can look great on the B but will require a low profile tire and ideally lowering of the suspension but you will feel the road and any bumps/holes more than a standard set up. A happier compromise without going into to much expense would be a 16" rim with a 45series tire. If your clever with your choice and use a 5 open spoke design then the rims look bigger and you keep your ride sweet.
Hope this helps
