There are several vendors, who ship systems with Debian or other distributions of GNU/Linux pre-installed. You might pay more for the privilege, but it does buy a level of peace of mind, since you can be sure that the hardware is well-supported by GNU/Linux.
Unfortunately, it's quite rare to find any vendor shipping new Motorola 680x0 machines at all.
Whether or not you are purchasing a system with Linux bundled, or even a used system, it is still important to check that your hardware is supported by the Linux kernel. Check if your hardware is listed in the references found above. Let your salesperson (if any) know that you're shopping for a Linux system. Support Linux-friendly hardware vendors.
Some hardware manufacturers simply won't tell us how to write drivers for their hardware. Others won't allow us access to the documentation without a non-disclosure agreement that would prevent us from releasing the Linux source code.
Another example is the proprietary hardware in the older Macintosh line. In fact, no specifications or documentation have ever been released for any Macintosh hardware, most notably the ADB controller (used by the mouse and keyboard), the floppy controller, and all acceleration and CLUT manipulation of the video hardware (though we do now support CLUT manipulation on nearly all internal video chips). In a nutshell, this explains why the Macintosh Linux port lags behind other Linux ports.
Since we haven't been granted access to the documentation on these devices, they simply won't work under Linux. You can help by asking the manufacturers of such hardware to release the documentation. If enough people ask, they will realize that the free software community is an important market.
If you ask for Parity RAM in a computer store, you'll probably get virtual parity memory modules instead of true parity ones. Virtual parity SIMMs can often (but not always) be distinguished because they only have one more chip than an equivalent non-parity SIMM, and that one extra chip is smaller than all the others. Virtual-parity SIMMs work exactly like non-parity memory. They can't tell you when you have a single-bit RAM error the way true-parity SIMMs do in a motherboard that implements parity. Don't ever pay more for a virtual-parity SIMM than a non-parity one. Do expect to pay a little more for true-parity SIMMs, because you are actually buying one extra bit of memory for every 8 bits.
If you want complete information on Motorola 680x0 RAM issues, and what is the best RAM to buy, see the PC Hardware FAQ.