Well the simplest is something like the KR808D-1 - a 2-piece unit
Vapor4Life's Vapor King is that model and they sell a lot of prefilled cartomizers (atomizer and juice cart in one disposable piece) - not the cheapest way to go but but can refill cartomizers some number of times (4-12?) until they either stop working or start tasting bad (assuming you don't run them totally dry which can shorten their life by singeing the stuffing). Their cartos are available in 0 nicotine as are their juices and the juices of most juice sites. Cartos hold more juice than carts (usually at least 2x as much). What may be a downside for you - it can give a pretty strong hit. If you want something you can drag hard on, this is probably not the unit for you. The vapor is warm because it's a cartomizer and it puts out a good deal of it.
A somewhat cheaper way to go with a bit more fussing would be a cartridge unit - something like a Joye 510 (find their cartridges hard to pull off though when new so if you are weak like me it might not be the best choice). It's a 3-piece unit with a atty you have to clean periodically (I swap attys weekly while I soak the other one in alcohol overnight then shake out and dry). With carts you have to fill more often (take the cart off and do a full fill into the stuffing or top it off with a couple of drops). I use a unit with even smaller carts - an M401 and I have to push the inner cart up with a paper clip to fill. The plastic carts can be reused over and over unless they get damaged or loose. The stuffing does not last as long - after 4-5 days I usually have to replace cart stuffing with blue foam stuffing which lasts much longer (strictlyejuice sells precut piece of blue foam for 510/901.)
If you want something more like smoking lights or ultralights, and can put up with the small carts and a paperclip to push them up from filling, you could consider an M401. I was an ultralights and lights smoker and I liked slow drags and savored them, then this is that kind of unit. Nice flavor, not a big hit. I replace the white cart stuffing with blue foam but I have to cut the foam myself.
Don't let anyone tell you you need a Riva or an Ego (a bigger unit) if you're not even a smoker now. Not unless you have a need for a super-long-lasting battery charge. If you are usually near AC power and can be charging one battery while you use another, then any size battery will work. If long battery life is important to you, consider Ego which is based on the 510.