As a 1st car, they are ideal.
I have a little 899cc one. It is the early model (1993 with the fuel cap on show, not behind the fuel flap like later models). It does 45mpg in London rush-hour traffic, and will still sit @ motorway speed limits if required.
I use mine for work as it uses less than £20 per month in fuel compared to nearer £60+ in the Saff, plus I can park it in places that other cars could only dream about.
Insurance for me (29 with partner on policy) is £110 FC. Tax is £110 for 12 months. 1st car owner would be probably around £500-£600 I imagine, so don't go getting one for £1000+, they ain't worth it.
In the event of an accident, they ain't all that, but most very small cars like that aren't.
Stay away from a base model (like mine) as they literally have no electric toys other than a rear wiper and heated rear window. An SX has electric windows and central locking, much better option.
Mine came to me FOC as it was the father-in-laws who drove it until he died. I am the 3rd owner and it has done 76k, mainly with no problems.
As far as rust goes, look under the back seat. A common rust point is near the rear seat belt anchorage point. IF it has gone there, then you have water leaks in through the boot, which could be anything. Chassis legs go, be wary of any with lots of rust on the 1/4 panels. Other than that, a good, regular service and they go almost forever.
Also, look out for doors that do not close properly, or drop slightly when opened. A mate used to work in the docks taking these and the Sportings off the ferry for new delivery. Out of every 100 off the boat, 5-6 would end up with twisted chassis' by the time they were parked as they would bend coming off the ramps almost sideways

The later Sciecento models didn't suffer this problem as the chassis was stronger....