I have a year 2k Td5 and have been "improving" on an ongoing basis. I have all sorts of material used in different places, which I buy, as and when I can afford.
The open cell 4mm fire retardant foam is cheep and easy to fix, with an appropriate high temperature spray adhesive. You can buy the self adhesive version, but I don't rate it as highly as plenty of adhesive. That's fitted to my new GPS Puma style bonnet in two layers.
I have fitted an Alpine system, with head unit, amp, six speakers and an Alpine under seat subwoofer. The speakers are fitted just behind the front driver and passage doors. I have built a long fitted box in 4mm ply that hold 2 9x6 Alpine speakers. The ply was very carefully cut with two boxes for the speakers. I then filled any gaps and used spray foam to fill the cavities. I also used the ply to cover both sides of the internal bulk head, fitted one (so far) amplified, sub woofer. I then filled the cavities with more spray foam.
I also fitted a plywood rear floor, wheel arch upsands and tops. Beneath the plywood I used expanded polystyrene foam sheet and bolted down with s/s .
In the headlining, I bought some foam mats from Liddle (the type that are used for floor excercise). Very cheap, easy to fitt with high temp spray adhesive. I them filler the gaps between the fron head lining with 6 mm close cell foam. I have still to finish the rear headlining, but intend to use the same product.
I did a huge amount of research before I undertook, but was totally confused by the end of the process. I am of the view, that anything that you can use to either absorb sound or eliminate vibration has to be a set in the right direction