i just looked at the diesel as it would pull better than the petrol with such a heavy car
In terms of the diesel you need to find one, go for a drive & then decide for yourself.
Entirely personally, I would go for a 2.4JTDm Brera because it looks better and you get the boot space, but maybe that's not a consideration for you. I've been living with the Alfa 2.4 diesel engine in various forms for over eleven years now and I reckon it's a classic engine, but YMMV. As always, you need a full service history and you should hold out for a years warranty as well.
These cars are very rare so you will probably go with whatever you can find but, if you have a choice, you might want to consider the following:
Wheels: 18" or 19" are more likely on a Spider, but there are plenty of Breras running around on 17" wheels. The larger sizes look nicer, but you pay more for the tyres and the ride gets harsher as the wheels get bigger.
ICE: The basic setup on the early 08 cars isn't very good, and the factory fit Sat Nav isn't up to much either. If you get a head unit that does MP3 disc playback and USB connectivity you're doing quite well, otherwise be prepared to fork out for an aftermarket setup if this is important to you.
I'm not sure there is much more to say - I don't have specific experience of the Spider, only the Brera upon which it is based.