I have been using one of those lightweight fillers for some time now; Ultralite, it's made by Siroflex or Sureseal I think. It comes in a tub ready to use and fills smallish cracks and fair sized ones. There are other makes as well, just pick up the tub and if it feels empty then that's the one.

Personally, I use Johnstones Trade undercoat and gloss. It goes on nice, covers well and dries fairly quickly.  Many decorators swear by Dulux Trade but I think it is overpriced and no better than a lot of other paints. Most painters have a favorite paint they tend to stick with.

People tend to turn their noses up at Leyland paint, but it is a fair price and I have no problem with it.

If you are competent with a paint brush then use trade paint, but if you are not so sure of your capabilities use ordinary paint from the sheds,
it will be easier to use.