Hi. I own the older model 520, so maybe I can be of some help.
The 500A is the first model with the new slimmer nacelles, it's fuel injected and has 260hp IO-470's. Solid direct drive engines whereas the previous models had the more finicky geared engines that had to be flown gentler and not throttle jockeyed around. The 500A's engines you can mistreat.
You will see roughly the same performance as I get on the 520, but the new nacelles gives you a little edge. I'm guessing somewhere between 160-170kts, knuckles to the wall. 30gph and upwards, give or take. Aero Commanders have a huge cabin/baggage and you'll run out of useable weight long before you run out of space.
There are three recurring wing spar AD's you need to verify that it is in compliance with.
AD 98-08-19 is the leading edge forward attach bracket inspection and panel due at 6000hrs. Cheap and relatively easy to do.
AD 94-04-13 is the forward spar cap inspection and this is due every 500hrs. Easy to inspect.
AD 94-04-14 is the spar cap replacement for galvanic corrosion. This is probably done, but make sure to check the logs as it's expensive.
If you're in the US, there's a guy called John Towner in Kansas who runs a freight operation with over 30 AC500's called Central Airlines. He's got some good STC's for deicing and other stuff, should you want to upgrade the aircraft. They also repair them. Or, should Stockton, CA, be closer, Morris Kernick's Commander services is the best and most experienced in these. In the UK, I don't know who services them, but there's a 680E still at Elstree so might be worth checking with that owner.
What can I say about Commanders that hasn't already been said? Obviously I'm biased, but I don't think you can get a better airplane for money. They fly beautifully - super stable IFR platforms (I've heard, I only fly VFR), great single engine performance, rugged, tough. They'll get into fields shorter than any other twin, rough, grass, gravel, whatever - they don't mind. Great visibility, roomy cockpit. Never have to worry about prop strikes. Great rudder authority and ailerons that never stall makes them a joy to fly. And safe.
Steering takes a little while to get used to and any hydraulic leaks should be fixed as everything on that plane is hydraulic. That's about it.