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If you want to tie yourself to the MAC you might also look at CSI Concepts3D. One of Ashlar's former movers and shakers is behind it (one of the better ACIS interface programmers by many accounts). I doubt it's very "assembly-centric", though.
PC based: I might look at Alibre. It appears to be the best deal going on a broad scope entry level mechanical design package.
Either of those will set you back one to two thousand US.
I don't believe you can legally buy any legitimate CAD software "used" or get discounts on old versions. Their new seat sales and upgrade revenues would go down the tubes without artificial market controls so they've worded their license agreements to preclude the possibility.
Most major developers offer student and / or personal use versions for one to two hundred. Pro/E personal edition is 250 per a recent advertisement. Pro/E is capable of taking you further than any of the rest, for my money. The question then becomes; do you need to go that far to do what you want to do?
Whatever route you decide to pursue; consider support. Low cost or free support resources will prove to be invaluable. You are not going to sit in a vacuum and become proficient with any sophisticated CAD software.
- - Good luck with it.
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