Trailblazers are a great vehicle. I agree with Terrie. You want to make sure the vehicle is structurally and mechanically safe before worrying about body work.
If the vehicle checks out and you are seeing some rust you want to replace start in a small area and see how severe it is. I know trailblazers have a problem with the drains which caused rust in some. What happens is water gets trapped in the door well and can't escape so it begins to rot.
The floors are typically where people start when restoring. The oldest trailblazer is from 2001. Personally if you are considering alot of work it might be too much $ to restore a 2001 vehicle. If it is minor touch ups then you might be ok. Also if a vehicle that new is rotten it may have suffered flood damage.
To answer your specific question start with the floors before doing doors/body work.
I think you may want to do some more investigating before taking on a big project like this!