So, am I right then that 18 ports will be the way to go?
yes but on the market, you'll have 16 i/o and the next one 21 i/o. A LED sacrifice can be done to save space. Choice is in your hand now!!!

I do not think there will be room for any onboard testing equipment for playback. That is fine for the recording pcb though.

What is this for the recording end of it?

"Edge connector with ICSP capability"
O.K. ICSP (in-Circuit-serial-programming) this can be used to program/update your PIC firmware. I was thinking about only 1 board who'll do all the stuff. Program your board to record, and after reprogram it for playback. Or something like that... i think that's comming a bit complicated a this point for you... but we can design/plan it together. As you want *one day* use surface mount stuff, i felt it was cheapest to build only 1 board who do everything.

With the 4000 serie, all we need : MISO, MOSI, SCLK, SS pins.

And all this can be done with basic and NOT pro versions of the picbasic?
For sure, since everything can also be done in assembler too, feel safe with PBC. We will use SPI/Microwire comms. Will be a bit more tricky than with the PBP SHIFTIN/SHIFTOUT but, nothing is impossible.