Sound deadening
There are a million ways to do this. You can sew the padding to the carpet if the carpet is to be removable or just glue the whole mess in there or.... . Don't forget drain holes in the foot boxes and any other place where water will pool.
Jute is the 'king' of padding but does not like to get wet a lot. Foam (called rebond if I recall correctly) deals with water better. Most don't pad at all, although a little on the tunnel is nice.
Do not use roofing ice sheild!
Insulating materials... any of the commercially available brands are good. You can do the whole cockpit or just certain areas. At least do the footboxes, trans tunnel and below the doors near the pipes. Best results is on the outside of the footboxes (and then cover with aluminum) and the bottom of the trans tunnel. It should be obvious where you put it under the doors!
Buy enough and have some mats made and put in snaps for them. Heal pads are nice in the mats and a pad on the trans tunnel right of the gas pedal is nice because it is a high wear area.
If you can find a spot for a dead pedal, no matter how small, put one in. Other nice touches are a heal brace and pedal stops for the gas and clutch and metal trim on the door sills.
Carpet that snaps in instead of glued in makes it nice for cleaning and drying and can conceal access panels to those hard to reach spots like shifter and top of trans and those pesky inboard brakes on jag rear ends. Access panels are a snap to make, take you favorite cutting tool (I like the rotary cutters for glass), cut the hole in the glass, back with aluminum sheet overlapping even for Dzus fastener and there you have it. Not a must but it can be a nice feature.