There are two things in your plan that you should revise.

You said that you want to start by printing things and selling them, you are putting the cart in front of the horse there... and the horse may end up biting you in the rear.
First learn the ins and outs of 3D printing and part modeling and design, if you don't you are bound to end up facing clients not satisfied with your work on one hand, and you'll find yourself facing unforeseen problems on the other; and if both happen at the same time you'll be in a stressful (if not economically damaging) position.
3D printing is not what it's cracked up to be, you can't just get a file, press Print and hey presto, ca-ching! It's better to realize that before you have clients knocking at your door with complains.

Second thing, your business plan is to basically appropriate the work of others; unless there's explicit permission for commercial use from the creators of the objects you plan to sell you are setting yourself up for, at best, a lot of flak from the community or at worst legal action.
To make things worse that is probably the least suitable and profitable business model for 3D printing, the strength of the concept is the ability to create custom objects tailored to the needs of people, and for that you must have modeling and design skills or hire someone to take care of that.