They certainly are a different experience. Lots of the same qualities, but the extra poke, especially in the lower gears is added fun. Also, if you try one with front disc brakes you'll appreciate the consistency in braking.
Original Abarth 500's do not seem to have any noticeable premium for mileage, so unlike a Ferrari you could use it well without harming your investment. Having said that, when using it you get the same fun with a good recreation and about £25K left in the Bank.
The one you link to looks to be in very good condition and is priced well given it is a genuine RHD. UK cars have tended to have more bodywork issues, so you can check sills , inner wings and front panels for signs of rust. None of this is major and is easily rectified. It all looks to be well specified with synchromesh gearbox, disc brake conversion and a suspension overhaul.
You don't often see engines over 700cc, so my only question would be reliability, but some of the more experienced people on here like Hobbler can wade in here.
My advice would be to see and drive a few and see what you like, don't like. If Cambridge is not too far, you are welcome to try mine out. Another alternative, if you like a bit of spannering, is to upgrade your existing 500. It's very easy to source the parts and recreate an Abarth/Giannini in the same way the crate upgrades were originally available.
I recently did exactly this and it was really enjoyable. I researched what went into the original Abarth upgrade kits, and the additional Abarth options like the dashboards, steering wheels, disc brakes etc. and then built a replica 695.
cheers, Steve
Btw Do you have an Rouse ford, called Andy Rouse, or actually are the Andy Rouse? I only ask, as when growing up in the mid 80's, I used to watch Andy picking up all those BTCC titles.