I can only answer the Eno question. It's a great hub in itself but fairly expensive. It's available in 126, 130 and 135mm. It's really just the end caps that are different. Same hub. The only sacrifice would be the rear brake can be difficult to set up. With the Eno the axle rotates in an offset circle around the axle bolts. This means there are two positions in the circle that the tension is correct. One above the axle bolts and one below. On my bike the brake reach was not enough to position the pads correctly in the lower position and the tire rubbed the brake in the upper position. So no rear brake for me, but I run fixed only so it's not a problem. Single speed I would want a rear brake.

I understand wanting to prolong an older bikes life by changing things up a bit. I would probable just use spacers on the free hub with a single cog and a adjust the tension by going up or down a tooth on the rear. Or just use a tensioner. Pretty simple really.