Should I plaster or just paint?

I would highly recommend plastering, but you would need to remove the existing wallpaper. Plastered surfaces are much better to look at and will give you a better finish, especially if you're painting.

There are a few options that you can do all depending on your budget first take all your paper of the walls and ceiling so that you could assess the state of your ceiling and walls first where the cracks are tap to hear if hollow on all effected areas if the plaster is sound as does not have a hollow sound then it may be ok to sand and fill, once dry sand and you could paint depending on your taste if the walls and ceilings are good and sound not damp you could tape all the crack pva or grit bond ceilings and walls and then plaster where needed if the ceilings and walls are hollow and damp then call you local plasterer for advice on what can be done

I would recommend plastering too. The finish will look much better then painting over it or trying to make it look good with filler and decorator caulk + repairing it will take some time and the cost will rise up

I would recommend plastering , plastering would fix any cracks,blamishes and any high or low dips on the walls and ceiling and give a nice smooth finish in which you can paint straight over without any worrys . 👍