Models Options

[Erikc]

As posted on the TES forums:
My question is this: how would i go about chopping up the current models? For example, i want to only use the bottom half of a certain interior piece, so that i can add my own ceiling height later. I was planning on doing this by extracting the current model, cutting off the irrelevent bits, and adding it in as a different model; the problem is i haven't the foggiest idea how to do this. What programs can i use to create 3d models (preferably free/cheap)? How would i get the data to them?