At this point in the market, I would HIGHLY recommend purchasing a 30W iStick over the 20W. The 30W is a HUGE improvement at roughly the same price-point. The 30W allows you to find your vape on practically any atty and fires pretty accurately.
That being said, I still have my 20W in rotation. I often run a PT2 w/AFC on the 20W. I currently have a 1.2Ω, 28g, 6/5 wrap, around a #4 bolt (approximately 2.3mm) and wicked with rayon. On the 20W I vape it around 3 - 3.5V; 13-18W on the 30W.
Your 1.8-2.0Ω rebuilt head will vape fine on either.