If you want a bargain performance car I can't see a problem with the lower cost end of the price range, as other have said (and I'm sure you are aware) they will either be imports, high mile and /or low spec uk cars. The other option is cat D's which are a different story, some don't mind them.
At £5.5k I'd imagine the value won't go down by a massive amount over the next couple of years, maybe a grand or so?
There are members on here with high mile cars (some over 100K) which are going very strong and are in great condition.
Just be aware that older cars will need bits replacing soon, and 350Z's do between 20-30mpg dependent on how much lead is in your feet