A six year old Mac is still serviceable, but kinda long in the tooth. The minimum for Lion is an Intel Core2 and 4 GB RAM. Does your MBP have that?
Software: We Use Vectorworks. Excellent product. Maybe more than your $500, but not much more. It has full 3D capability right out of the box. You might find a used package that will run fine on your machine. I am almost certain VW 12.5, 2008, and 2009 will all run okay. 2010, and 2011 might. 2012, maybe not.
Other professional grade software available for the OS X are AutoCAD (probably no older than Snow Leopard) and ArchiCAD (older versions may be available used)
Semi Pro CAD is available from TurboCAD and VisiCAD. At least one of them (not sure which) is in the $100 to $150 range.
Good luck. 
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