I did about half of my PPL training in a diesel PA28 from an 800m farm strip. It was absolutely fine. I did my first solo and a couple of solo nav ex's in it and then my instructor bought an avgas 172 which took me all of 2 hours to convert to before I was sent of for more solo nav ex.
If it's cheaper then there is no reason not to do your PPL in the diesle model. Having since flown an Avgas PA28, the only real difference is a slight reduction in climb rate and you do kind of have to haul the diesel version off the ground at take off.
As I mentioned, I did my PPL out of a farm strip. I now fly from Norwich airport which is Class D and full ATC and it's no problem at all. I wouldn't worry about learning from a commercial airport as it just adds more cost. And grass runways are a bit more forgiving when you are learning how to land
