What’s the minimum recommended compressor HP/PSI to spray paint

First off, I'm no expert on compressor/air type sprayers. Most of the time, these type sprayers are used for very thin paints such as automotive, lacquers etc. They also have fairly small paint reservoirs attached to the gun itself. They tend to put out a lot of over spray and should be used in a controlled area like a spray booth with proper ventilation. I never see this type of sprayer on building jobs.

The airless sprayer, such as the one you noted, use a high pressure pump to push paint through the hose to the gun. They deliver a good pattern (depending on tip selected), are easily controlled and since they don't mix air with the product, have far less atomazation of paint in the air. the other huge advantage is that the pick-up hose can be put directly into your 1 or 5 gallon can of paint. Follow the directions on priming and cleaning, but it is easy. I personally have a Graco model 17 and love it. It is a professional grade sprayer and retails around $450-$500. Just two weeks ago, I and a helper painted and back-rolled a complete two story house and detached garage in one day.(very long day, however!) This saved 3 or 4 extra days of labor and the results were fantastic. We shot both semi-trans stain on the house and matching color solid stain on the garage. This sprayer handled both products easily. (I always use apx 10 to 15% flowtrol with thicker solid latex based products)

The only disadvantage of this type of sprayer is on very small jobs. It takes about a pint or more of paint to fill a 25 foot hose and closer to 1/2 gallon for a 50 ft hose when priming the unit. This amount product is usually wasted at the end of the job when you get ready to clean the pump.

From the type of jobs you plan to do, I would recommend the airless sprayer hands down.

You'll have some waste with spraying. Considerably more than with a brush or roller, anyhow. Find the square footage---both sides---and divide 325 into it. You'll get about that amount, 325 square feet per gallon, if you're careful. You will likely need more than one coat…

Also, make sure the surface has been well prepared by removing loose paint and sanding rust spots. You'll want a good primer, preferably oil-based, on those bare spots.