Steam Summer Cards 2017

From a value-to-exp standpoint there is no point in crafting any, you should sell all of them.
However, since they are limited (as is the case for every sale) you might want to craft a few just to have the exclusive badge.
However, the event badges don't max at 5, they go on to infinity. With the first 10 levels having distinct looks, then later on upgrades in the tens, hundreds and thousands. Most people will never go that far anyway, but you'll have to decide where to stop.
Yes, the card values will only go down and down - both for foils and normals.