I've been lurking on here for a while,I can't decide on which engine to go with. I've got about 5.5k to spend, I do roughly 8k miles a year ,12miles each way to work and back.
At that price point, you'll likely be buying a car that's outside of manufacturers warranty and you should also consider the potential cost of unscheduled repairs. The 1.2 is by a long way the safest option here; potential DPF issues with the diesel and uniair &/or turbo issues with the TA can't just be dismissed - any of these could give you an unexpected four figure bill. Also routine replacement items such as brake parts and filters will be about half the price for the 1.2. If you really want or need to keep costs to a minimum, consider a Panda instead; they're far better value and, dare I say it, more likely to have been driven in a sedate manner.
If not a diesel it will be a 1.2, just for cheap road tax and mpg.
Seems to me you've got there already. The TA also has cheap road tax, but may cost you more in fuel, despite what the official figures may have you believe. The 1.2 will also likely be cheaper to insure; whether this makes much difference depends on your personal circumstances.
Whatever you buy, keep about £1000 aside for first year repairs; the cars are basically sound, but on the sort of age vehicle you'll be buying, you could easily need to spend money on the brakes, suspension, hatch wiring, battery & tyres - most of which won't be coverable under any sort of warranty - and it all adds up.
Also check the finance offers on new cars; on 8k/yr, it just might make more sense to get a factory-fresh one on a 3yr pcp.
With current Fiat offers, class-leading residuals and likely repair costs, the 500 is a car that IMO is better bought new and traded away at the end of warranty (unless you're handy with spanners; the 1.2 in particular is a good car for DIY maintenance and the parts are generally cheap).