Any vagcom/KKL cable should work. Go for UK supplied to be safe. The mod involves soldering a short piece of wire from pin 2 to pin 7. I managed it with hands like bunchesof bananas.
On the software front, alfadiag or fiatecuscan can do fault code reading and diagnostics inc. reset of warning lights.
Fiatecuscan is £42 for the licensed version which you'll need to reset codes and Alfa Diag is £46.
Both do the same jobs, FiatEcuScan is still being developed with updates very frequently made to include additional fucntions. Also,Yani the fella who wrote it takes requests for features if something isn't covered.(I have a copy for my Fait Stilo Abarth, it works really well)
I haven't used AlfaDiag in a few years, but used it on my old 156 sele and GTV. It works well and does the job. But I believe isn't being developed further in any meaningful way any more.
Some cables from EBay come with cracked software on CD, mainly VAG and Ford stuff, but some have basic fiat and alfa code reading and reset abilities.
A full setup of cable and either software will cost about £60