You really shouldn't have any need to change the car for a while now that you have given it a good going-over. As you have seen rust will be the thing to watch especially as the car is now over 10 years old, but isolated areas can be cleaned up and protected by a simple squirt of waxy spray. I can't imagine that little car trapping mud etc. in areas that will rot unobserved.
I've always found that Fiat underseal and seam sealer (along the floors and sills) deteriorates secretly after 10 or so years, and water seeps underneath and starts rusting if you're not careful.
But otherwise that model was a great little car and probably the last classic, simple, small Fiat that we will see (especially with the original door mirrors that were replaced with the daft "elephants ear" style later ones.
I've driven your model and the new one as hire cars in Italy, the current version is nice and grown up and civilised but it's essentially a more practical and conventional variant of the 500.
