Hello and welcome!
Check out Overtons for marine grade stereo and speakers and the Bimini top.
I would recommend marine grade stereo componenets (head unit, speakers, wiring) since you probably get a little spray.
Crutchfields is also a good option. They can send you customized install instructions and walk you through it if needed. Their customer support is top notch.
If you wanted to buy locallly, try Custom Sounds...they carry marine grade and are competitive with West Marine.
I wouldn't take my boat in for a simple stereo install. As long as you are not installing amplifiers and need extra batteries, I think you can DIY.
I wouldn't glue down hydro turf. I think you would be happier with some snap in. Capitol City Upholstery carries several lines of boat carpet. You can find snap in instructions on-line.
Your rig is Coast Guard rated for "X" amount of passengers. Adding seats / capacity is not a good idea. I wouldn't breach the floor screwing in pedestals that might rip out or you could hit some wiring or your fuel tank. It would also impact your performance....your boat seating is designed for specific weight points. If you added seating up front for example, your rig might not be able to plane out or would list really bad. IMO, adding seats just isn't a good idea. It would also be an insurance risk and might not be covered. If you need better seats while anchoring, maybe get a couple of folding camping chairs.....just don't use these while underway.
For the bimini, check out overtons.
http://search.overtons.com/?Ns=p_image_sort|1||p_purchased|1&Ne=1000&N=2070&cnam...