As a way of adding a different perspective to the postings before me
my 'take' on adjustable coilovers is that it is more for
track guys and they're expensive but since you're considering them let's increase the budget...
One of the 'good things' about an A500 is that because the standard 500 is essentially no different to its Abarth 'brother' you can use the same upgrades. I would be partial to the
Konis FSD and here's a
set for £545. The
review on them indicates that they are best suited to fast road use and for me would be better than 'fiddling' with the settings.
On the Abarth section there's a
thread on the limitations of what you can do with the 500 but a point raised by Maxi is that because your model is a '09 it has no anti-roll bar (it would be better if it had). A particular brand shock is mentioned in that thread and I have attached a 'robbed' picture with an Eibach spring (Pro). Maxi has mentioned Bilsteins - an excellent shock but it often comes down to personal preference. The FSDs above will work with standard A500 springs and can be picked up cheaply.
I have also attached a picture from the US Fiat web site of the rear suspension showing that the suspension travel mentioned by grimwau is only 25mm. Some with coil overs have removed the bump stops but IMHO that's a bit risky. Some have 'cut' them leaving a 'stub' and others have replaced the stock ones with Fiat coupe/Alfa 156 bump stops which are half the size (again robbed from a different forum).
On the exhaust modification the Sports cat that I posted will be of no benefit unless you change the manifold - so now we are into a full exhaust system since there is only one silcencer on the 1.2.
Powerflow got a mention since your based in the UK (acknowledgement to Trackdayqueen) and if you check the Supersprint site it shows the same exhaust for the 1.2 as it does for the 1.4
. If you go for the backbox there would appear to be a preference for the sound of a Raga over a Magneti Marelli so you would need to hear both before you spend your money. Flames as mentioned by RUI wouldn't come out the back exhaust like the 20V Turbos.
On the wheels and going up a size to 17s personally for me the weight of them is important - I didn't like the heaviness of the 17 inch 10 spokes on the A500 - here's some indication of
weight differences.
I would suggest that you set a budget on mods on the car and stick to it.