I wouldn't buy a model pack, because I like to make everything myself, even if it's not as good. I don't like to be in a position where I have to say "look at this game I made. Oh, except that, and that......and that."
There's more personal achievement if it is done yourself. However, other people will want to spen all their time on gameplay and programming, and want some nice visuals that they can just plug in and use. Unfortunately, I'm not one of them.
The thing that could pursuade me, though, are items that no-one enjoys making. Designing and making main characters is fun, and really cool. However, making 7 different types of chest, 6 crates and 32 different-looking npc's is pretty dull and repetative work. That's the kind of stuff I think may sell.