its pretty straight forward, seats you can probably pick up $300 a set, unless you throw down for the recaros.
I have a set of blue leather seats im trying to unload from my 325i, 4 door, back seat has headrests and arm rests. Not to many people are big on blue though. But then you wont have to change your door panels.
So check ebay, seats go cheap there, or look at roadfly.org.
When you swap seats its really easy, 4 bolts for each front seat, back bottem pops off if you pull on the edge, then there are 2 bolts near the interection of the 2 halves on the sides of the car. Undo though and pull the top part of the seat upwards to unclip it from the frame.
Door panels there are a couple screws near the door handles I cant quite remember where right now but look in the bently it has it all.
Carpet, just redye what you have.