A used copy of Autocad isn't a particularly good solution, since it doesn't give you an upgrade path & it costs plenty.
Unless you are familiar with CAD applications, choose carefully: o TurboCAD mentioned is very good o TotalCAD is another - easier to learn/use but more limited
There is a personal preference between those 2 applications, comes down to which people feel is more intuitive vs learning.
AutoCADs benefit is industry standard plug-ins - from design elements through to design & virtual-prototyping tools. That said, standalone apps often offer more than AutoCAD can integrate, forming a workflow chain of engineering apps costing 150k$+. Unless the application is commercial, look other than AutoCAD.
TotalCAD supposedly has a shorter working curve, but that said a lot is still counter-intuitive - although perhaps less so than TurboCAD.