I would have thought a BIS in good condition would be well suited to a 30-miles-a-day round trip. Most baby Fiats in the UK had short lives due to bodywork corrosion rather than mechanical failure, plus the (at the time) increasing availability of larger, better performance cars at ever cheaper prices. If you plan to use your BIS as a daily driver, it should be fine provided it has been properly serviced during its past life, you keep it properly and regularly serviced, and when it is repaired, it is repaired properly with the right parts and not bodged to save time and money.
Getting some parts may take time as they are nor readily available on every high street, so allow up to a week for a part to arrive by post from Germany (or Slough!). Equally, the cars will be unknown to the younger generations of garage mechanics, so not every garage will know how to repair them to the best level of expertise.
Your plan will be fine when the car is running, but you need to plan also for when it might not be. And this is true for any car, even a brand new Porsche (of which I happened to see one on the back of a recovery truck on the M25 this very afternoon!).
So you need to know which friendly, knowledgeable and affordable garage will fix the car for you (if you cannot fix it yourself), and you need to have a Plan B transport arrangement (the bus, a lift from a mate, working from home etc) for the days you may have to be off the road while waiting for parts or repairs. But be sensible about estimating this. If the car is used regularly for reasonably long runs as you describe it will benefit from being regularly warm, and aired, and with all the moving parts like bearings and suspension actually being moved rather than just stood still for 20 years.
You will doubtless find residual weaknesses from age (for example, water hoses that may only have done 10K miles but may now be 20 years old) become apparent, but as these get fixed you may well find reliability improves substantially, and quite quickly.
It would obviously also be prudent to have membership of a rescue service to get the car to a place where it can be fixed if it breaks down mid-journey. My own recommendation based on over 40 years of belonging to such organisations from the AA downwards is Britannia Rescue. They cover the DRIVER, not the car, so they will rescue you whatever you are in. They got me home once when I was merely a passenger in a car with no other rescue cover, but they've also bailed me out with things as diverse as simply running out of petrol (how embarrassing was THAT !!) to a kit car 5 miles from home and a classic car failing in the snow over Shap at midnight. And all for around £80/year.
IIRC, the Bis suffered only one major design flaw and that was in the location of the thermostat. Others more expert than me will correct me, but I seem to recall there is a well-documented solution to this by rearranging the hoses so the thermostat is at a different position in the cooling circuit. But don't quote me on that !
