UGFs will such all the waste under the panel. Periodically you'll need to clean that out. Depending on what sort of panel you have and how much suction your siphon has you might be just able to clear the gravel to the side, place your siphon on top of the panel and it will such the gunk from that area out.
Have you read up on cycling a tank? If not, thats where you should start.
If you use the filter you will have to make partial water changes, 20%-30% about once a week. Vacuuming your gravel with a siphon at this time is generally the best thing to do.
You should never have to replace your gravel unless you get tired of it. With a UGF, your gravel is your biological filtration and replacing it will cause the tank to cycle again.
I would just try the pump that came with the tank for now and see how much current it creates in the tank. If your fish is getting thrown around, you can get a different type of filter, a smaller pump, or a valve that will control how much pressure the pump puts out.