Using building lumber/??

Since weathering would not be a concern, how well the result would hold up depends almost entirely on how well you construct it.

SYP is hard and heavy. It is also "fat" which means there is a lot of resin in it which means it is a PITA to sand because the abrasive loads quickly. Here in central Florida, SYP is commonly used but usually only in load bearing applications such as headers. The more normal construction wood is "white wood" which can be fir, hemlock, spruce or pine.

Construction lumber here is #2 common. (If there were such a thing as #5 common, I'm sure that would be the grade of choice :); #2 common has lots of knots and other imperfections. If you want your project to look good when finished you have to get all the surfaces that are to be painted smooth and flat. Here is what I would suggest to do that...

1. Cut your boards to size trying to minimize lumber imperfections.
2. Use Bondo or other auto body filler to fill major dings, knots and other deficiencies. Those fillers are a mixture of talc and polyester resin. To use them, one scoops out some filler then adds a few drops of catalyst (which comes with the filler). Mix the two - THOROUGHLY - and apply with a putty knife, overfilling slightly. It will set up quickly but takes 20-30 minutes to get totally hard. Once it is set up but still rubbery, use a sharp chisel to cut off any excess. Never get catalyst or catalyzed resin into the can of un-catalyzed else it will set up and be ruined.
3. Sand everything well with #120 paper and dust them off. Examine each piece well - a glancing light helps - for surface imperfections. When you find them (you will) fill them with automotive glazing compound. You can get it at NAPA or other such store. It is talc in lacquer, comes in a sizeable tube for -. It is meant only for minor deficiencies, not deep ones. If you still have deep ones, use Bondo again; use the glazing compound on minor ones. It doesn't set up like Bondo, it dries by evaporation.
4. Sand again with #120 to get all repairs smooth and flat. Then sand again with #150 or #180.
5. At this point, you are ready to prime and paint. If you can do so before assembling - priming especially - do so; if not, assemble.
6. Use an oil primer. Brush it on in the direction of the board's length; i.e., don't leave cross grain brush marks. Let it dry for a couple of days and sand with #180 or better. Most primers don't sand real well nowadays and yet it is necessary to get it smooth as top coat paint will look no better than the surface to which it is applied. If the paper loads too much, try hand wet sanding with a rubber block sander and silicone carbide paper.
7. Apply two coats of your finish paint; again, avoid cross grain brush marks. I'd suggest you NOT use glossy as it will show imperfections very readily. Eggshell or - at most - semi-gloss is better. If you are able to spray primer and finish, you should have an excellent job; if not, you should have a good one.

What's the grade stamp on it? The codes in the US have four general classifications, "Spruce/Pine/Fir", "Douglas fir-larch", "hem-fir", and "Southern Pine" with different allowables for each grade of each, from Select Structural down to #3. The highest allowables are for Douglas Fir-Larch. It should be stamped for one of those categories.