Building a simple PC - looking for a CPU
If you want to find a device with a minimal amount of on-board peripherals and an external memory / data bus then "MPU" or "Microprocessor" is a good term to search for on supplier websites. For example the following is the embedded microprocessor page at Digikey:

http://www.digikey.com/product-search/en/integrated-circuits-ics/embedded-microprocessors/

If you use the parametric search on that page the problem you'll find is that a lot of simpler ones that fall roughly inside those specs are obsolete. Most of the current more capable ones run at a higher clock rate and come in BGA packages that would be harder to work with and are generally more complex. That's not suprising because most modern commercial designs would either benefit from the integration of a microcontroller or require something at the higher-end where design and manufacturing complexity is less of an issue.

I'd probably stick with a microcontroller similar to the one you've already found with an external data bus  / SDRAM controller to attach external RAM. You could also do the memory interfacing directly using I/O pins. For the peripherals you can always choose what on-chip peripherals you use versus what you add externally. You'll probably find plenty of challenges such as attaching the VGA LCD to keep it interesting.