Welcome and I assume you are talking about an unregulated mod that can somehow be switched between series mode and parallel mode. I can't make any recommendations about pre-built coils, but if you could explain what kind of vape you like I'm sure others will jump in to help with that.
On an unregulated mod the first thing you have to look at is always what the manufacturer of the mod says about the safety. As long as you're not exceeding those limitations listed in the official specs of the mod, you can treat it like a mech mod and calculate the safety the same way you would calculate it for a mech mod (that is, series mech mod or parallel mech mod, depending on which choice you made for battery configuration). It doesn't have to be .4 ohms and above to be safe because, for starters, it can never be completely safe, and, the .4 is assuming you're using a series mech mod with two 20 amp batteries. You can go all the way down to a .24 and still be within the recommended battery safety limit for a series mech mod if using two Samsung 30T batteries, or a .09 for a parallel mech mod using two of these same Samsung 30T batteries. (Still assuming this is within spec of what the mod can handle according to the manufacturer's official data.) The math is simple. For a series mech mod using two batteries, just double the voltage. So 4.2 volts times 2 batteries equals 8.4 volts. Divided by .24 ohms equals 35 amps, which is the CDR of the Samsung 30T. For a parallel mech mod using two batteries, 4.2 volts divided by 1.5 batteries divided by .09 ohms equals 35 amps. The reason why you're counting 1.5 batteries is to put a 25% extra safety margin on each battery to account for battery imbalance in a parallel mech mod so 2 batteries minus 25% equals 1.5 batteries. (Experienced mech users would not take 25% because 15% is reasonable enough, but taking 25% buys you more safety to start using a parallel mech when you are still new to using a parallel mech, or in this case an unregulated parallel mod.) Unregulated mods can have built-in safety features, but personally I would never rely on those features without having done a lot of strong research on them so just treat it like a mech and follow the specs first. The Samsung 30T would be my pick... for safety and performance.
