Just adding to what 2_Seek_U has said, if you're looking for bolt-on options here's what you can do with changing over items between VR-VS.
Interior
The front seats in all VR-VS models use the same rails and mounting points so you can use whatever you like from VR-VS. If you want to go with something from VT onwards you'll need to keep your VR rails.
The dash I wouldn't recomend going with anything that has climate control as standard (Statesman/Caprice/Calais etc) because of what you'll need to keep in the front console (climate control surrounds for instance). You can convert to climate control but it's a majorly expensive exercise, and the general consensus is it's not worth it for the hassle required. Unfortunately that limits you somewhat with what models you can grab stuff from, so you'll be stuck with base models (Exec, Acclaim etc or Clubsport if you want some badges).
Front and rear door trims you can use whatever you like, including Statesman/Caprice, they're all interchangeable.
Assuming your rear seat hasn't had any mods done to it, you can choose whatever you like from the short wheelbase models (Exec all the way through to Calais). The Statesman/Caprice rear seats mount in a different way and have a totally different construction and would require extensive modifications.
Exterior
You can change the front end over to VR-VS Statesman/Caprice, and for that you'll need Statesman/Caprice bonnet, grille, L/H & R/H guards and bumper. The Stato sideskirts while longer than standard do have a little bit of adjustment in them, but still won't be anywhere near long enough for your needs. The rear end you'll have to leave the same (i.e. any VR-VS short wheelbase model).
If you want to change the whole look over to a VR-VS clubsport kit it will all bolt up - again except for the sideskirts. You may be able to butcher 2 pairs of fibreglass sideskirts into 1 long pair but I'd leave that to the pro's.
Hope this helps.
Mark.