Here are Logic Pro 7 System Requirements
• Macintosh computer with PowerPC G4 or faster processor (G5 or dual G4 processors recommended).
• PowerPC G5 and Gigabit Ethernet connectivity for Logic Node applications.
• Mac OS X v10.3 or later.
• 512MB of RAM.
• DVD drive for software installation.
• Available USB port for XSKey (copy protection).
• Low-latency multi-I/O audio hardware and MIDI interface recommended.
• 4GB of available hard drive space.
So I'd say any G5 Mac can do the job, and a Power Mac G5 will give you the best expansion possibilities. These should be fairly inexpensive on eBay now.
That's an aging set up, though – Logic Pro 7 was released in 2004. But if it still works...