Deciding which paint to use has gotten much easier now that acrylic latexes have pushed oil-based paints almost to extinction. The acrylics offer superior performance (they don't harden with age, the way oils do, so they move and breathe without blistering), they don't mildew as readily, and they emit fewer VOCs, so they comply with new air-quality regulations. They also work over both oil- and water-based primers.
If there is any peeling — and there usually is somewhere — it's essential to scrape off any loose paint. This doesn't just mean the parts that are already separated from the wood and ready to fall off on their own. Each spot where paint has peeled loose needs to be scraped thoroughly until you can no longer get the sharp corner of a putty knife under any of the surrounding edges of paint. Then, the bare spot needs to be sanded until the paint edges are smooth. Westminster House Painting