That will be a good size for your Ball.

A rhp is not much like uth device. A rhp is overhead and delivers heat is a wide area which corresponds to the size and wattage of the panel. They still need a tstat and in my opinion need a good tstat and not an el cheapo.

As to needing both a rhp and uth I really cant tell you. If you have a cool home it is possible that having both is best. That is something you would have to figure out in your home. The size and wattage of the panel will change the output of the panel and that vs. the temp of the room and the temp you are trying to reach in the cage will be the determining factors of if it will work for you.

If the cage is a good deal and in good shape I don't see reason not to get it, even if you don't want to use the rhp. Take it out and sell it if you find you don't need it.

I have several rhp in different cages. For my arboreal snakes they are a must but for my other snakes Im pretty sure I could do without them. Many times in the warmer months I will cut them off. I only use mine to boost my cage temps a bit so I can keep my oil filled heater on 75F instead of 77-80F. I only use those for the arboreal snakes to create a hotspot. They others are only to boost the ambient temps a bit and all of those snakes have a uth.