Get ready for some sticker shock my friend: replacing the cushions with new custom-made ones is ridiculously expensive. However, that's not what you asked.
Used any good closed cell foam, like a polystyrene. Closed cell foams are much more resistant to the absorption of moisture, so they will be less likely to get mildew and hold a funk. As for fabrics, you can go with Sunbrella, but if these are interior cushions, that seems a bit of overkill to me. Any good grade synthetic fabric will do the trick. I would pick a pattern that is on the light side (to avoid making the cabin seem dark), but with a pattern that will hide dirt and stains.
You should also consider having the bottom side of the cushions covered with something other than the top side fabric. I have seen it done with something like a solid vinyl, and with a mesh fabric. Two completely opposite approaches, both with their pluses and minuses.
One other practical tip: when I was looking for quotes to replace the cushions in my boat, I found the marine specialty shops to be universally higher-priced than my friendly neighborhood upholsterer. By at least 10%, and in one case, almost 30%. Btw, the cheapest quote I got to replace the seven interior cushions in my Oday 23 was over $2000. In fact, one of the marine shops who quoted me a price of about $2200 said that he knew it was a lot of money relative to the value of the boat, but that he only pretty much broke even on these small boat jobs. This was 10 years ago, and I can't believe it would be cheaper now.
Now you know why you always see those "make your own cushions" articles in the sailing mags.