Hi, and welcome.
The Mistic seems to be quite reasonable on their kits, but around where I live the cartos are priced like the Blus: 5 for $15. When I researched what connection type that they might be, many said that they looked like 808D, and one said emphatically that they were 901-type. Those two types are the same threading, but have different airflow paths which make some of the four possible combinations bad. It may be worth trying out a 901 or 808d carto, so you can order blanks at more reasonable prices.
Box mods are pretty cool, though. I would suggest reading up on the forum about battery safety before using one, though. I'm building my second one this weekend. My first box mod was a variation of the
Puck e cig, where I used a 4AAA box for the batteries and a 2AAA box for the electronics, leaving the kill switch on the 4AAA box. It's pretty small, and I made a 'slug' that I could put in place of one of the batteries to change it from 5v to 3.75v.
It is larger and heavier than a small li-ion box mod would be, but I prefer to deal with Ni-Mh batteries that I can buy in most stores (because they are not scared to sell those...) The battery life is great on the AAA one, but the AAs boast around 3x the capacity of the AAAs, so I'm building a 3AA version, using a 1AA for the guts, and it's barely any bigger than the 4AAA one. I used 510 connectors from GoodProphets for both of them, instead of the homemade 901 connector. Either is a little heavy for a T-shirt pocket, but fine for a heavier shirt or jacket.
Anyway, I decided not to buy the Mistic kit, and got an 1100mah eGo-type kit (510 style). While the battery life is amazing on these, they are annoyingly long. Put a CE2 XL carto on one of those, and it sticks out of my shirt pocket 2.5 inches. I'm probably going to buy some 650mah Rivas for away from home.
If you ever get into DIY liquid, I'd recommend trying Capella's flavor drops. Good luck with your ordered flavors! That's something that can take a lot of trials to find what you like, and from whom.