For 2D stuff such as floorplans and such, AutoCAD is what I'd go with though I haven't used it since my freshmen year of school.
For 3D, the two big dogs that come to mind are SolidWorks and ProEngineer. I've used both but my preference is for SolidWorks because 1) it's more user friendly and I don't need the power of ProE for what I'm doing and 2) every company I've worked for uses Solidworks.
You could easily get either program after some buccaneering on the internet and probably teach yourself the basics with tutorials.
Alternatively, you could send it to me (mechanical engineer) and I'll give ya a good rate
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