Hi,
I bought an 05 turbo-diesel sport in March and have a had a number of things go wrong with it, most of which seem to be common. The bearings are rubbish and will often go at this mileage. I've had the off-side front one go, and now the n/s one may be going. They are also prone to rust on the rear wheel arches so check for signs (or previous repairs) very carefully. Mine needed a very expensive cambelt change but I think you are right, the petrol is chain driven. The Suction Control Valve went, but again that is diesel related. I think you are doing the right thing going for a petrol, I wouldn't buy another Mazda diesel. Also mine's a Sport with the fancy Xenon headlights which are a nightmare and have cost me hundreds to get working, so avoid them if you can.
So all I have to offer for buying a petrol is to check the bearings and the wheel arches, and avoid models with Xenon headlights. Hope this helps.