The only Macs Parallels 7 doesn't run on are those with PPC CPUs (Apple stop producing them in 2006), and the only 2 32-bit only Intel CPUs Apple as used, Intel Core Solo and Intel Core Duo (not to be confused with Intel Core 2 Duo), which were only produced in 2006 and 2007 (afaik).
So, any model manufactured after 2007 should run Parallels 7 just fine.
Also, make sure it has at least 4 GB RAM.
