Some thoughts about the Sturmey 8 speeds:
-the disc version (x-rk8) is spaced at 134mm, and spreading aluminum frames is risky. There is also a drum version called the x-rd8, which is spaced at 132mm.
-i'm not sure you have a disc boss on that frame; there are bolt-on adaptors if you want to try that route, but i'm pretty sure your spacing is going to be a deal-breaker for any disc-ready IGH, which are spaced @135mm pretty much universally, to be compatible with disc-ready frames.
-the x-rf8 is available in two OLDs: 134 and 120. You could almost certainly space-out the 120mm version to 125 and run it (the axle is 175mm long), but then you'd need to find a way to run a rim brake, as the x-rf8 is a brakeless hub.
-the main challenge with the Sturmey 8 speeds is that the lowest gear is the direct drive 1:1 ratio, so you'll need to run a huge rear sprocket and/or a tiny front sprocket, b/c all 7 of the other gears are overdrive. The Shimano 8 speed, by comparison has the direct gear in 5th, with 4 underdrive gears and 3 ovedrive gears, so you can gear it like a regular singlespeed or 3speed and it will provide a useful range.
The biggest rear sprocket they make for the Sturmey 8 speed is a 25t (there are also 23 and 20t sprockets; the 8-speed hub takes a different kind of sprocket than the "Standard" 3-splines that mot other Sturmey-Archers, SRAM, and Shimano IGHs take...). If you run the 25t rear with the stock 44t chainwheel, you'll have a range of 46" though 148" gears, with 30% steps between 1st/2nd and 7th/8th, and 14% jumps between all the rest. Basically, that puts your 6th gear at a very high 100", and 7th and 8th will be virtually unusable under most cruising conditions. For comparison, a stock 1909 3speed with the Nexus is running gears of 38", 52", and 71"....
If your frame was spaced at 135mm, i'd suggest goign for a Nuvinci, which has up to 380% range, with stepless changes in ratios (it's a CVT) and direct drive occurring in the lower-end of the range--- it goes from 0.5 underdrive to 1.9 overdriv for the latest n380 hub. But, really, your frame's material is limiting you somewhat in terms of what hubs you can run. I'd still say you're best off with an s-rc3 or an AWC, although Sturmey-Archer does make a new x-rd4 with a drum brake that is spaced exactly at 125mm. Shame it runs a rotary gear selector, instead of an elegant indicator chain...
For the life of me, i'll never understand why ppl want to put those plasticky Shimano clickboxes on bikes that are s'posed to look retro. Indicator chains and fulcrum clips just look the part more, plus Sturmey-Acher is far more user-friendly.
