Oooh! Lots of things to think about here! Most people, especially when buying a brand new 500, will go to a Fiat dealer. Obvious that one! Clearly the dealer can be your most local one, or you can travel further afield, very much depends on what deals the dealers are offering, though to be honest, if you look a the Fiat UK website, they quote their standard 'on the road' pricing, so you clearly have to do the best deal you can with whatever dealer you intend buying from.
Then there's the pre-reg option. Basically, buying a 'brand new' car that has already been registered by the dealer. Pros and cons of doing this, main 'con' being you won't technically be the 'first owner', the garage will and some people just won't get themselves over that aspect and truly believe they will lose loads of money buying that way, which in my past experience, isn't entirely the case. The main pro, is that the car can come with a really hefty discount, but! the con with this is, the car could be several months older or indeed over a year older, i.e. from when it was actually built, to when you buy it. I know this from experience. Our TA that we bought from a dealer 150 miles away from us last November, was in fact 14 months old when we drove it away with only 10 delivery miles on the clock. You must ask what the assignment date or build date of the vehicle is prior to purchase! That being said, it is still a good way of buying a car. We saved about 3.5k over buying a vehicle built to order.
Buying second hand from a non franchised dealer. This can be fraught with problems if you don't do your research. One of our forum members recently went through every car buyers worst nightmare. Bought a car with a false history, accident damaged, repaired poorly and sold on. Unfortunately, not even a HPI check picked that one up! Thankfully, it all worked out right in the end, but it goes to show that you must do you homework before you part with any cash. And on the cash subject, always, always pay a part of the deposit (£100 or over) on a credit card. But whatever you do, make sure you inspect the service history of any pre-owned car, if necessary, pay for an independent inspection etc. The hassle and heartache that can result of not doing so, can be horrendously stressful.
When checking ads for second hand 500's or indeed any car, make sure you scan the add for Cat C or Cad D 'write off' repaired. I personally wouldn't touch one, but people do buy them and they are obviously quite a bit cheaper than cars that haven't had an accident.
Buy from an auction house - only for the very brave and experienced car buyer!
So as I've already mentioned, I bought from a franchise dealer miles away from me after scanning Autotrader. The deal worked well for me and I have no complaints (other than the fact it was my own stupid fault for not asking the build date of the car I bought!) but other than that, a fine little car. You should obviously, go and inspect the car first, unless you are a risky little beggar like me who is happy to buy blind over the 'phone lol!
All I would say though, is don't be afraid to travel countrywide if you have the ability to do so. The savings can be huge and of course you don't have to worry about going back to the franchise you bought the car from for servicing or warranty work because your local franchise can do that for you. I wish you the best of luck in your search!
Oh, yes, forgot to mention the buying privately option. Can be a good way of buying a decent second hand car, but again is fraught with potential problems. It is not unknown for unscrupulous dealers for instance, to flog off cars this way. You absolutely must take the greatest of care when buying this route. Clearly, not all genuine owners selling cars are bad, but there are some who somehow 'forget' to tell the buyer something might need replacing, or they've had accident damage they've repaired outside of insurance, indeed a whole load of stuff potentially they might not tell you and the very reason why I personally haven't bought a car this way for over 20 years. Plus you can't pay using a credit card and your rights as a consumer are not the same as if buying from a franchised dealer or good independent dealer.