1. Depends - if you are prepared to share a van it can be as cheap as $5 pp (one way). If you want a taxi all to yourselves, I believe it is $18 per cab (one way). Prices are prominantly posted at the taxi dispatch area.
2. Really not a lot of head-banging music or loud boozing around. Pedro's (right next to CO resort - so lots of nude people) has a band several days a week. When we stopped there for ribs, the band was pretty good. Going north along the beach, progressively less nudity. Bikini Beach (middle of the beach) is nice - clean,nice loungers, outdoor showers but so are lots of other places. My advice just get dropped off and wander along the beach until you find a place that appeals. North end of beach is least crowded.
3. Once again- it depends on the place. My understanding is that a couple of places near the north end offer free loungers - if you have lunch there - sorry cannot recollect names.
4. Once again- it depends - some places have very attentive servers who will come down to your lounger regularly to see if you require drinks or food. At others - it is just faster to go to the bar/restaurant.
5. I hate to keep saying this but it depends - at Pedro's (one of the cheapest places )you can get ribs and rice and cole slaw for less than $10. I believe Baywatch nearby is comparable. In my experience the places around the middle of the beach are the most expensive and the places at either extreme end are cheaper. Prices range from $ less than $10 to over $20. Overall foodwise, St Martin is NOTcheap but the food is usually good quality.
Some places include a free drink with lounger rental, some do not.
Lounger rental cost varies. When we were there last we got 3 well-padded comfortable loungers, one umbrella, one little table for around $20.
Nudity: toplessness - very common; full nudity - obviously most prevalent closer to south end of the beach near the CO resort; least prevalent north end. However ANYWHERE on thebeach you may see one or two nude people walking by. Believe me it really is no big deal.
I hope this helps.