I can easily hear the basement C0/2 and smoke detector from my bedroom on the upper level. so interconnecting is NOT a must but it's more of a nice to have, I might consider the wireless inter-connecting models regardless, but then it would only benefit me if i replace all 3 smoke/co2 alarms at the same time, otherwise i will have a mix and mash of different brands and products. either way, when you buy a previously owned house you NEVER know and should never trust what the original homeowner left you with, he could easily say it's 3 years old, and in my case the upstairs wired smoke alarm was 23 years old instead and was giving all kinds of false alarms.
I have existing junction boxes on all 3 floors which i can wire them into so needing an electrician or a permit is likely not necessary. Really its only the basement one which is the one that urgently needs to be replaced, right now there is no C02 alarm there at all, I can re-locate one of the other ones there for the time being because I heard the furnace room is likely the main source of C0/2 leaks