Which SSD do I need

Hi there, about to buy a late 2012 with a Apple original 750gb blade SSD but it's just too much for me, I'll rather have a 120gb SSD + 3tb HDD then sell then 750gb SSD so I was wondering:

1) can I install any regular SATA HDD like in previous iMac models?

2) which kind of SSD should I be looking for, is it M2, PCIe or both are the same? There are SO MANY different connections for this sort of flash drive even between Macs - I googled and it's crazy, it changes A LOT between Mac Pro, MacBook Pro, MacBook Air, iMac...

I'm kinda lost here, which connection and size (lengthwise) I should be looking for an blade SSD compatible with the 2012 late iMac model?

Thanks in advance!! :)