Picking a Stain is a very personal choice. You can go to your local paint store or big box store and look at samples of stained wood. Your local paint store can also guide you. You could also pick up some mahogany and stain & finish some samples to see what you like. Spar is actually a finish coat that goes over the stain or raw wood to protect it from the elements. I do not believe there is any finish that you will not have to redo every few years because the sun is hard on finishes.
Good luck
Michael

Don't use Spar varnish. It isn't tough enough.

I have a carbon composite front door. After staining it, I applied 5 coats of Zar Exterior Poly to it. That was several years ago and it looks like the day I put it on, even tho the dogs sometimes scratch at the door. It is extremely hard. Even steel-wooling between coats was a tough way to go because you couldn't see where you had used the steel wool and where you hadn't--it's that hard. It also has excellent UV ray protection.

You might want to use a gel stain. They are so much more managable than a wiping stain. I used a wipe stain on mine but fauxed it to really look like wood and it turned out great. But if you haven't had a lot of practice in manipulating stain, gel is a good way to go.