Welcome, Ian. I lived in Pasadena in the late 80s/early 90s when I acquired many of my 18Fiats, but I'm in the SF Bay Area now. But, that's neither here nor there. Well, actually, it is there and not here. But I digress.
Anyway, yes, this looks like it would be a good project car, but I'm kinda thinking more in the $1000 to $1500 range. And yes, I do agree with you about back registration fees, as I got dinged with a $500 bill 30 years ago when I didn't know this little piece of DMV trivia.
It's actually been so long since I've owned a fuel injection spider that I can't offer much help on the smog and missing hose connection issues. Maybe others can.
The interior can be fixed up, but parts for this are not cheap. A good cleaning, new carpet set, and some seat covers would be the cheap solution, but you can get yourself into $3,000 worth of parts and labor without too much trouble. Also figure on $500 to $800 for a new top, plus labor (unless you do it yourself). I think you also might have a crack in the right rear taillight lens, which could set you back a couple hundred bucks unless you just leave it or fill in the crack with some colored plastic repair stuff. As for paint jobs, a really good paint job on a Fiat can run up to $6,000 in CA. One of the joys of this state... Of course, there are cheaper jobs available (Maaco?) and I've even seen some pretty decent paint jobs done in a driveway with a good air compressor and professional paint gun. Just don't pick a windy day.
I'd also take a look at why the rear bumper is pushed in on the driver's side. Accident?
As for surface rust, that's probably OK as Fiats rust if they sense frozen water vapor molecules on the back side of one of Jupiter's moons. Just make sure there isn't structural rust around the underside, shock towers, seat mounts, etc.
Hope this helps.
-Bryan