I don't know the grade of the material but it is magnetic which means it is a ferritic stainless with a high ferrous content. It welds okay but is thin and very easy to burn through.
Re Nelmo's comment above on engine emissions for MOT purposes. You have the advantage of a Q plate which means the emissions test is a visual only. As long a it's not smoking excessively you will be ok.
My mark 3a has inboard front suspension. I kept the original dampers but ditched the springs for 120 lbs one. Transformed the car totally. Better ride and handling. Can't remember where I got them or how much they were but well worth doing.
Regards Dave
Without an air filter you run the risk of dirt being sucked into the carb and the engine. If you can fit a filter somehow then you should. A simple foam type would do.