That's a good choice for what you're considering, in my opinion. I've used it before on a snowblower. It's brutally tough stuff. Don't expect to sand it after a day - it'll destroy the sandpaper. You can spray it but it's rather toxic, being a moisture-cured urethane. An air-supplied respirator should be considered a necessity for spraying. I agree with Glen. POR15 is the easiest brush-applied paint I have ever used. It flows out exceptionally well.
If you don't want the flat finish after exposure to sunlight, then topcoating can be done. I've successfully avoided using the special POR 15 primer by using epoxy primer applied within the short time window (4 hrs as I recall) of the POR15, then topcoating with an automotive acrylic urethane. Make sure you download all the detailed instructions from the POR15 website and follow the preparation procedures (in addition to wire brushing the loose rust off) and the time windows between coats carefully. I found it difficult to get two coats of POR15 plus a coat of epoxy primer on in the same day to meet all the time windows but it can be done if you do all the prep work beforehand. Still makes for a long day though.