My very first car i built was a mk3. I fitted a mk4 xr3i interior if i remember correctly the door cards need a bit cut out for the door handle.
I think the mk4 doors are the same so you could use them. As for the dash i think it fitted straight in. I used the mk4 loom but you could easy splice the mk4 electricals into the mk3. The back seats i can't remember if there is a bit for bolting the middle bracket on or i welded the bracket.
You can easily fit mk4 bumpers needs a few bits cut and filed they look quite nice with the mk3 grille. When i done this my mate had written of an xr3i at the time so i had access to all the parts apart from engine and box.
Brings back some good memories.