Interior repair is easy. Just take out everything, scrape the asphalt like sound deadening off, paint it, then cover it with cheap carpet.
To save the most money then doing the simple stuff like weight reduction and minor aero mods (grille block, passenger mirror delete) are your best bet. A lot of the bigger mods end up costing enough that the payback on them from better mileage takes years so from a strictly money saving point of view they are not worth it.
In my opinion the first thing would be strip the interior down and take the sound deadening stuff off to help keep it from rusting as fast and put some good paint on the floor. Then either replace and restore the interior or do away with it and cover the metal with carpet. I normally ran my xfi with only the drivers seat and removed all other seats/seatbelts. It should be no more than a day of messing with the interior to get it nice looking, the cars are pretty simple to take apart and work on.
Other than the basic interior work and a good tune up I would probably just drive the car for a while and make sure it is getting at least 50mpg before doing any mods. Without a baseline you are kind of shooting in the dark with the mods. The weight reduction is just to improve acceleration and really won't give you a big increase in mileage.