why not just tie them all together off of the remote out on your head unit? these devices take very little power on the remote line itself, you probably don't even need a relay.
However if you wanted to use one, just get any SPST or SPDT 12V relay (you only need < 1A contact current, so any relay will do), and then just hook it up like you described.
As I reread your post, I see that you also want to power an inverter off of this. How big is your inverter? You can probably just get any SPST or SPDT 12V relay at 20A or 30A which will power a 240W or 360W inverter, respectively. Use the remote line from your head unit to switch this relay as well. I prefer using the head unit because the computer is only on when the radio is on (which really is always) and then I have an easy method of turning it off from the front while keeping the ACC line high, but you could just as easily power it off of the ACC line from the car.