I have fitted Terra Firma Springs and Shocks to my 300tdi and I couldn't be happier. The rear springs are the progressive ones e.g. tighter coils at one end. The front springs should are a constant width coil. The rears are also longer than the fronts when they are stood side by side. Not sure why you have a mixture of gas and oil shocks? If you need advice I can recommend speaking to JJ (John Jennings) at Frog Island 4x4, JJ is also a 300 disco owner and obviously has Terra Firma kit fitted.
I hope this helps.
Chris