Low Cost 3D CAD

Hello,

I just joined the group and thought someone here could help me.

I have an idea for a product that I believe I could sell in a decent
quantity (100-200/month) made out of aluminum or hard plastic
depending on costs, etc.

I have the design in my head and want to prepare a 3D design using a
rather inexpensise CAD program.

I have a couple of questions:
1)What software should I use to generate a file that can later be
read by a small manufacturer to produce these parts? (The design is
fairly simple but not trivial. I have a background in engineering
and have used a high-end package (Ideas) in a previous job about 10
years ago so I'm sure I could learn the software fairly easily)

2)Would it be better to outsource the design services as well? (If
this is a one-time development effort then I wouldn't want to invest
a lot of money in a CAD package that would collect dust.)

I have heard of some manufacturing outfits that do rapid prototyping
that like to use STL files, but I also know that the AutoCAD DXF
format is still very popular.

Thanks for your help.
Dave