What are you planning to do with the Mac?
There is a tonne of stuff that will run under System 7. Let's face it, System 7 had a very long life (nearly 6 years) while Mac OS 8/9 had a life of under 4 years. Couple that with software having better backwards compatability back then, and you will find that System 7 is a viable platform to work from. A close runner up would be Mac OS 9 due to it having Carbon (unlike 8.1 IIRC) and due to it having a modern version of Carbon (unlike 8.6 IIRC). Anyways, choosing the desired role of the machine will allow you to choose applications, and choosing applications will allow you to choose an OS, and choosing the OS will constrain your hardware options.
The one requirement that you did mention was transferring files. What will you be transferring files from/to? Overall, vintage desktop Macs will provide more options: most of them can support ethernet, almost all of them have floppy drives, almost all of them have serial ports, and you can sneakernet hard drives if you have something really big to deal with. Almost all of them will support CD burners too. A lot of the laptops can handle that sort of thing as well, but it usually means buying extra hardware or special cables -- thus adding to the cost.