In my experience I would not recommend getting jellies. Though, I've only raised Moon Jellies and have no experience with the upside downs.
The moon jellies were in a kriesel tank and although we started them out on pureed Formula One we had to move to encapsulated Naup. Larvae that we hatched and maintained. Their tank must be siphon cleaned with something extremely tiny so as not to bump them, they are incredibly fragile.
We maintained extremely high water quality and still within five weeks two had permanently everted themselves, one had gotten bumped with the pipet we were feeding them with and had a tear in its bell.
After eight weeks we had two left out of the six given to us by the aquarium that I volunteer at, (the place I kept them was at the Marine Aquarium Management building at my college).
Only a few days later the jellies at the aquarium (numbering in the hundreds) were dying off in droves with no explanation. We later found it was a very strange and rare plankton bloom coming in from the water we use from the bay beside the facility.
The classes jellies didnt have any problem with the mixed salt however, and grew beautifully.
I certainly dont want to discourage you but it is good to know the risks, I would say that they are medium-difficulty to keep, but you must accept a failure rate of some kind, namely deaths with no explanation, if you want to keep them. They are great creatures and its nice to hear that others are interested in them.