I’ve got my concerns about how long this will last!
How long it lasts is really up to you!
As a 63 plate Twinair owner, mine was originally bought as a commute car through my LTD company to do a 100mile a day commute. At the time I could right off £17500 worth of income and corrosponding corporation tax in the first year due to the low emissions of this engine. I did this commute for a while and then left that contract.
Besides this though I loved the Fiat and it's drive way more than our BMW X3, which is probably a little more capable (although not much) but vastly more costly!
The Twinair 500 is perfectly capable! I think it is one of the more capable small cars.
Think of the engine design as half a high performance 4 cylinder 1.8 and it should last with regular oil changes to what you would expect from a larger car.
Mine is now just up to 30000miles. I did replace the top mounts and all 4 shocks with Monroe gas units (the Fiat rear shocks are particularly bad) at 19000miles and it feels like it did when it was new. In fact a year on those replacement shocks still feel great!
Mine has always had an oil service at 6000miles. In fact the Alfa/Fiat specialist I used to take it to in the UK commented, that they were seeing a number of TwinAirs with stretched warn or damaged cam chains due to the oil servicing being left to 18000miles. He suggested 10000mile max for oil services.
So it depends if your prepared to spend a bit keeping up on the maintenance!
I would suggest oil changes at 9 or 10000miles. Changing the shocks gave me that new car feeling back and made a huge difference to the handling and drive of the car. By 18000miles the bushes on the Fiat rear shocks were warn and the rear end felt sloppy!
Mine has also had regular washing (every week when knocking up the mileage) and a polish twice a year. The rear suspension was also inspected and treated to wax oil at 20000miles and the car still looks like a 6 month old 500!
I would guess at 5 years old and 36000miles your not going to get much of a trade in on it. WeBuyAnyCar offered me £5800 in January (then 3 years old) as we were moving to Spain - that felt like giving it away so we imported it. On the other side of the argument, that is still low mileage and if you have looked after the car it will be good for a few more years motoring!
I would guess over the coming miles its going to need - Brake disks, pads, front suspension top mounts and shocks - assuming that none of this has been done so far. Still way less expensive than the monthly payments or upfront cost of a new car!