Smok 2.5 ohms ,dual coil cartos, 3 holes is what I use 95%. You will have to modify the Nova to get anywhere near the performance of a DC carto but cartos need to be changed anywhere from 2-7days, I usually get 4 or 5 days out of one .
Madvapes has a good selection.
You can always drip a little into the carto filler on those occasions when you have something thicker and it's not keeping up.
2 ohms single will be too hot, 3 ohms single will be ok
2,2.5 and 3 ohms dual will all be ok
Oops, oh ya the battery life, for example a 3 ohms single coil and a 3 ohms dual are "seen" by the battery/PCB as simply a 3 ohms load, could be 100 coils @3 ohms, same load on battery. (EDIT: But since it is two higher ohm coils the user is going to probably increase the voltage which will decrease battery life.)
4 ohms single coil cartos are out there but far and few between, 4 ohms DC cartos if you could find any would need something like 8 volts, we're talking about two 8 ohms coil inside a 4 ohms DC carto