There is no need for the cap you mention - PicoUPS offer a 1.2A charger / switchover board at the low end and higher capacity boards as well to take care of the UPS part. Instead of a lossy diode or other approaches, they use MOSFETs that act very quickly. Read up on the info they have on the site, I was impressed.
The PicoUPS power supplies have wide DC input ranges and plug directly into a standard motherboard plug. So, I am not sure what you need other than to figure out the peak loads, the required components and a credit card... 