Good idea - without the middle bar, it looks like a Nova.
I've found that fibreglass-reinforced filler is best - it's stronger and less likely to crack. Use the stuff that's sold as 'metal fix' - grey colour, fibre-reinforced. You'll be able to fill the back of the grille bar/base and it will be really strong.
Sanding it is probably best done dry and definitely with a block, with sandpaper starting at 80 grit, then (roughly) doubling in grit each time - that way, you don't end up with scratches that show up later. So, 80 grit, then 180 (say), then 280 grit, and 400 grit. Or whatever sandpaper you have that's close to these!
To sort out any pinholes or blemishes, you'll want to give it several coats of primer/filler (thick primer). The primer/filler from an aerosol can should be given at least 24 hours to dry between two thick-ish coats. So it's going to take a couple of days, but then you'll be able to block-sand with wet 600 grit sandpaper and get a really even base for your colour coats. Which, again, have to be put on fairly thinly with plenty of time for each coat to dry.
The reason for the extra drying time is to avoid shrinkage problems. If the paint underneath isn't properly dry, the top surface will shrink as solvents evaporate over the longer term.
All this is assuming you don't have 2K primer and a compressor/spraygun handy
2K paint has the advantage of being thicker and harder (oo-err) than aerosol paint.
Also, though I haven't seen your FIAT badge (the one on the right), you might like to know that they can be bought new through a FIAT dealer and it's well worth it for the crisp new look (the blue plastic ages badly and the chrome comes off!)
-Alex
