If it's a 3.5ml. DCT by smoketech, it comes with one carto and you'll need longer XL type Boge or Smoketech Cartos for replacements.
The standard one to use is a 1.5ohm Dual coil Smoketech Stainless Steel carto. You can get them pre-punched or punch them yourself. Some people use Boge XL's as well. The main thing is that it's long enough. A standard length carto is too short.
You're having a textbook case of "Paralysis by Analysis".

The cure is to pull the trigger.
Oh yeah, A tank is for
vaping your favorite juice. the one you want to vape all day (unless you have a bunch of tanks, one for each flavor).
You still need atty's to find out what juice you like best. You still want other cartos in case you don't want to vape the same thing all day. If you have to do away with something, scrap the 3ohm Boges.
You want other cartos to use for other flavors. One tank carto = one flavor.
One regular carto = one flavor.
Once you put a flavor in a carto, whether it's in a tank or not, it's pretty much committed to that flavor.
You can clean cartos but it's a PITA and usually doesn't work all that well. I don't bother.