Unless you want to go back to bare metal, definitely recommend a RANDOM orbital samder. I bought a 125mm one but found it virtually impossible to buy wet and dry discs (rather than dry ones), they all seem to be for 150mm ones, which is what I would go for. Buy discs in bulk, they get eaten up at a fair rate.
I've got on well with rolling using a 4" foam roller, then laying off with a really good brush, as wide as you can handle. Purdy as recommended by Phil Speight are really good. But whatever the method, the paint needs to flow well, you may need to use an additive in warm weather.