I'd forget about 18490 and get the 18650 extension... longer lasting, and easier to find a pv that fits batts.
there are many li-ion 18650's. so you'll be better getting button tops ..or better yet stick with recommended IMR's.
provari is easier to use than darwin imo... and the customer service on both are equal imo, and the warranties.
i've had both. the darwin can take you to higher voltage vaping than the vari, (might service LR's too, not sure) but for 2 to 4.5 Ohm the vari hits the large spectrum.
some little glitches in darwin design (no drip well, button hard to press, leaks onto the dial) is a small price to pay for the extra watt range, but for a state of the art, easy to use, reliable regulated power VV, Provari's my choice.