Hi! I'm still here, just not very active. It's my Birdy in that picture. I still use it and am happy with the Alfine 11. I sometimes build my own wheels, but in this case I went with a professional builder to save time. So I don't know what spoke lengths are used. Spoke brand and thickness is not important here, just go with a common reputable brand and a normal dimension, not crazy thin ones. You probably wouldn't find super thin spokes in so short lengths anyway.

For Alfine 11 the choices are afaik: a) colour (black or alu), b) number of spoke holes (36 or 32 I think, I use 32 which I recommend on such a small wheel), and c) cable shifted or electronic Di2. I use cable shifted. Electronic costs more and you need to recharge the battery, but some will prefer that given how sensitive the mechanical one is to wrong adjustment. I think my model is SG-S7001-11, but just go with whichever model is current. As you see in the link Winfried posted below your post, the dimensions are the same for all those model versions.

I have the Shimano Alfine chain tensioner CT-S500 which like the hub can be had in black or alu. Many brands will surely work fine. I would recommend one with 2 idler wheels, not 1 like the CT-S510, because 2 gives much better chain management when folding the bike, and also better chain wrap around the rear cog. You could save money by using a regular derailer like the one you already have. Just remove the shifter, and use a few cm long piece of shifter wire that you cut just long enough to position the derailer in the right place for a straight chainline. If the derailer has a shift adjustment barrel, this should be quite easy to set up. Though a dedicated single speed tensioner probably weighs less and may be less sensitive to be damaged. You will need a narrow type chain. 8 or 9 speed is fine. Single speed chains are also fine, possibly better, as long as it's the narrow kind, 3/32" and not the massive 1/8.

You will of course have your own opinion on what the best gearing ratio is. Either the Alfine cog or the front chainring can be replaced to give a ratio you are happy with. You will want a straight chainline for the best chain life. Some Alfine cogs can be flipped to adjust chainline, but if not your best bet is to adjust the chainring with chainring spacers. They are tiny, cheap, easy to use and weigh nothing.

There are two small tricks worth being aware of, when it comes to the shift cable routing, if you choose the cable shifted version (if Di2, I have no idea). You route the shift cable through the frame, same as the original one. You can in fact use the same cable and housing as you already have mounted, unless they are worn out, because with Alfine you'll be shortening them. The Alfine hub has an arm that the cable housing attaches to. This arm should be positioned close to the hole on the underside of the middle of the right chainstay. The hub has coloured anti-rotation washers, specifically for left and right, which force the arm to be in a certain position. As I recall, I had to swap left and right to get the right position for the Birdy. Pull the cable housing out of the hole, and cut it down to the right length to attach to the arm.

Second: In this position, the chain would very slightly rub the Alfine arm, which isn't good. But as it happens, the chainstay has a threaded hole right next to the one used by the mudguard stay, which is in the perfect position to screw in a short bolt from the inside of the stay, so the head of the bolt pulls the Alfine arm outwards, away from the chain, just 1 mm or so is all you need. You could bend the arm a little instead, but the bolt solution is more precise. This has been working perfectly ever since. I can take a picture if you'd like.

Finally, Alfine 11 adjustment. Yes, as many people say, it is finicky. But when you get it right, it works very well. Start with the description in the manual, where you shift to 6th gear and adjust it so that two yellow dots align. But if you get skipping gears then, you adjust it a little bit in either direction until you find the sweet spot. This is for me a point where the yellow dots are 1-2 mm apart.

Be prepared for the oil change, the first should be done after 1000 km, then more rarely after that. It's very easy to do by following the clear instructions you get with the oil change kit. Or you could hand it to your local bike shop, but do make sure they know what they're doing. If they just squirt grease in there like a common 3-speed hub would have, they might ruin it.

Alfine 8 is cheaper, easier to adjust, and more solid, from what I read. If you can live with the smaller range, go for that. But for me I needed the range of the 11-speed, and I am happy with it. If I was buying today, I would consider the Di2 option, especially if it is possible to hook the battery up to my hub dynamo somehow. But all in all it's a great Birdy setup. Happy rides!