The first thing to do is draw a block diagram.
Key elements will include:
* A linecard for each extension, and the exchange (CO) lines;
* The switching matrix;
* The tone/anouncement generator;
* A central controller.

As you're starting from scratch, you shouldn't try to cram everything onto a single processor - that just adds unnecessary complication. It's far easier to at least start with a processor on each linecard, a processor for the tone/anouncement generator, etc.

"what are the simplest features that should be placed in this PBX. Like Voice Messages..."

The simplest features are the basic call handling & progress tones - voice messages, DISA, etc are advanced features. Get the basics working first before you start on the bells & whistles!!

Remember that you dont't actually need any processors or even electronics at all - it can be (and was) all done purely electro-mechanically!