Good fan speed for solder fumes
It depends on how much area you want to cover. I generally see under-bench systems with vent hoses in the range of 150-200CFM. The desk top ones can have lower flow rates as they usually sit closer to the work, where as the under bench systems draw more air as they're meant to have the vents sit further away. Keep in mind depending on how aggressive your filter is you may need to get a stronger fan to deal with the resistance presented by your filter.

If you want an idea as to what commercially available units use for a sense of scale, look at vendor sites. They usually specify the flow rates for their products. Some suggestions to get you started would be PACE, and Hakko.

You should only need one fan.