Its very difficult to say whether my commute was similar but it was around the same miles and a mix of motorway and surburban crawl between traffic lights. MPG was consistently between 23 and 24, worse in the winter from cold starts.
Where to buy from depends on your budget and therefore the age of cars you're looking at. Buying privately from here is likely to land you a well looked after example that you can trace the history of. Alternatively, bigphil, one of our traders may be able to help you find a good example - what he doesn't know about Zeds isn't worth knowing and his reputation is impecable.
Personally, I'd probably keep to Nissan stealers during the warranty period (its not strictly necessary but Nissan aftercare is pants and warranty work will be much easier to sort if you have a Nissan service history) but after that I would avoid them like the plague and would go to local performance car specialist. Wouldn't let the local greas-monkey near it though....
HTH