MC68000 Modern Equivalent (or can/should I use NRND/EOL chips)
The natural successor of the 68000 range of CPUs are the ColdFire microcontrollers designed for embedded platforms. They have sort of the same ISA, but some instructions are removed and some have been added. This means that they are mostly binary compatible, at least in user mode, but some instructions have to be trapped and emulated if you want to run completely unmodified 68000-code. The removed instructions are notably a lot of the byte/word addressing modes, meaning that it is more focused on 32-bit applications. They also have some very nice new instructions, for example the EMAC unit for doing DSP operations.

It is unclear to me what you mean to be the difference between an MPU and an MCU. I assume you mean that you want to attach external RAM, which is certainly possible with a lot of the ColdFire MPUs and MCUs. There are too many devices to list here, and you haven't given much to filter on, so you just have to search for a device that suits your needs.

I'm currently designing a computer based on the MC68060 CPU, long gone. I expect to source the few I need on the second hand market. If you're just building a handful, I don't see why you should worry about ICs which are marked end-of-life.