I still maintain (issues with later cars aside) that the 1.2 is probably the best engine in the whole range at what it does.
I'd agree, and it's probably the most reliable and longest lived engine in the whole range too. Unless you run it short of coolant or allow the sump pan to rust through, breakdowns due to engine issues are almost unheard of.
The 1.4 engine is essentially the same as the 1.4 engine in the 100HP, so if you don't like the power delivery characteristics of that car, you likely won't like the power delivery of the 1.4 500 either. It also has the same turning circle issues in 1.4 form.
After 145k, there are many things which could be well worn, but the 1.2 engine is a long long way down the list.
The 500 carries a huge fashion premium for its looks, even when well used, so at that price point, I'd put my money into a Panda instead; however you measure it, you'll likely get a lot more car for your money.
The Euro 4 1.2 used in the Dynamic Eco Panda pre the 2010/2011 changeover is probably the best variant of the 1.2 ever made, but that engine never made it into the 500. That got the later Euro5 engine from launch, although it was only homologated to Euro 5 with the 2010 model year changeover, when it additionally gained the rear beam stiffening & softer springs which together significantly improved the ride.
The Euro5 1.2 is slightly less flexible and has less torque lower down than the Euro4 engine it replaced. It did gain an extra 9bhp in peak power, but you'll have to go to 4000rpm to find it.
If I'd set my heart on an older 500, I'd at least get one with the later type of rear beam (or modify it later - the parts can be had from breakers easily enough now).