Welcome to the forum.
Your starter most likely has corrosion from age and moisture. Must use a "Marine" starter because they are in an enclosed engine compartment, and Marine Starters are ignition protected (meaning they have spark arrestor screens) to prevent igniting gas fumes. If you have time now, you could remove your starter and have it rebuilt.
Seats can be cleaned and protected with products from "303 products" such as 303 aerospace protectant
http://www.acehardware.com/product/index.jsp?productId=1420353 Mr Clean Magic Erasers can remove stubborn stains and marks in vinyl, but they are slightly abrasive, so don't overuse them. Lysol can help kill mold/mildew without resorting to chlorine based products that will eat the stitching.
All of your questions have been discussed in different threads here if you do a little searching. Many threads on sound systems, cleaning products, buffing and waxing, engine maintenance, etc.
We will need pictures of your boat, seats, flooring, engine, drive so we can give you advise specific to your boat. Pictures must be posted to another sight (such as photobucket or dropbox), and linked to here. Max photo size is usually 1024 x 768 pixels. Here is a tutorial on posting pictures on iFourWinns:
http://www.smwebhead.com/phpBB3/viewtopic.php?f=9&t=1877
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"Knot Easy" 2000 Horizon 240 Volvo 5.7GS /SX
tow: 2017 Honda PILOT EXL-AWD
prev. boats:
'87 Chaparral 198CXL 4.3 OMC Cobra
'69 Jetstar 16ft Ski Boat, 115hp Yamaha
'68 Aluminum Jon Boat, 3hp Sears
'64 Water Wings