If you were fine to live with out PWM i would go with a Scythe Gentle Typhoon, they make them in a wide range of values, i own 5x 1850rpm AP15 (if i remember correctly), i can undervolt them about 500rpm fine, and its design with a real ball bearing should be better for a horizontal placement, but its not as quiet as Scythe Slipstream or Nexus Basic. In the past, a lot of years back, Scythe SFlex were recommeded over Slipstreams for horizontal placement due to the design of bearing.... I had 3 sflex 1600rpm on a notebook cooler for couple of years (horizontal) and they were fine, why do i mention this..... because the Scythe Kama Flex 120mm PWM Fan (SA1225FDB12H-P)
is almost identical to my old sflex, just its pwm (300-1600rpm), i have 5 running in my HTPCMI (check sig). one is horizontal on the GPU, and its doing fine, and they are running at 250-300rpm, but you can set to whatever you want, as long as you mobo allow it, bad news is availability, its almost out of stock in the usual places... but amazon with 3rd party sellers have some and here is another here
seems to have them in stock. Heres a couple of pics for you to see,
Btw i also own 2x PLPS (check sig micromi build), and they are wonderful fans, but i havent tested horizontal, but this would also be a good pick, as long as you dont want super low speeds, like the kama flex can.