It depends on what your usage would be. If you do use it a lot for phone calls and texts then pay as you go would soon be ate up. On a contract you could get a package that gives you enough minutes and texts for what you would comfortably use. Or on orange/EE you can get magic numbers so that others on same network, then the calls are free. I don't know about other networks, they may do the same so it depends what your family/friends use and who you call most.
As for the phone, try to get one which is close to the one you have been using imo, as trying to find your way around one you don't know very well can be frustrating. Mine has a smart screen but also buttons to text so that it doesn't get thrown through a window.
It is complicated. Have you no young person to take with you to explain better? I take my son with me who is more savvy with these sorts of things. 