Welcome to the forum Andrew.
To answer your questions:
The 1.4 has more go than the 1.2. Also easier to find on the market, so I would choose that.
The auto gearbox provided serviced correctly is reliable. However, the manual I would suggest would be the more trouble free of the two.
The cat is up in the engine bay, and thefts have not been reported on the forum. Sadly, it is the MK1 that has the cat in a prone position, and thefts do occur.
Clutch? Well, buying a used car the life of the clutch will depend on its previous use. However, my car has 135k on the clock and is on the original clutch and exhaust.
You're on the money with regular oil changes and the timing chain. My Jazz has always had regular servicing, Honda for the first 8 stamps. In my ownership, it averages out at around every 7 or 8k miles mostly diy servicing. Cheap and easy to change too. At the moment I'm using Castrol 5w30 A5/B5 magnatec oil, in a Ford branded bottle. But little Jazz has had Comma oil and others in the past. The grade is more important that the brand imho. The timing chain makes no noise and is not an issue that comes up on the forum.
I've done >50k in under 4yrs in my car, and it hasn't missed a beat. It's only needed servicing per schedule, together with tyres and brakes as any car would.
It's a fine choice. Only drawback in my opinion is interior noise above 60mph on long journeys.
If you want to read up, manual is here:
https://hondafitjazz.com/manual3/index.htmlHTH