1:08 p.m. on December 14, 2011 (EST)
It isn't all that far to Little River, which has some excellent climbing, or even over into Georgia to Stone Mountain. I used to go over there when I lived in Jackson, MS (although northwestern Arkansas is better). Where is this 600 foot rappel in MS? Vicksburg has dirt cliffs, but not that tall. And for most of the state, that high a rappel would put you below sea level.
Anyway, to answer your question - based on more than a few years of climbing, and being an avid reader of each year's edition of Accidents in North American Mountaineering, I would strongly advise you to forget about the alternatives you mention. No way I would personally buy someone else's used or logged-out rope. You won't know the history of the rope - how it was used, any major falls, how it was stored. Rope loses strength just sitting in storage. Even a few years ago, the UIAA and CE tests were showing that in 4 years in proper storage conditions, rope loses half its strength. Advances in the materials and technology have allowed some companies to get that up to 7 years.
If you are using the rope only for rappelling, a static line designed for rappelling is a good choice, and it is much cheaper than a dynamic. But if you are going to use it for climbing, even top-roping, you definitely should spring for the extra bucks for a dynamic rope.
You are worried about $200? What is your life worth? That rope is literally your life-line. $200 is a pretty cheap life and disability insurance premium. As for substituting yacht line, keep in mind that rope is designed and manufactured for a particular use. The kinds of things a yacht or any marine line is used for are very different from climbing uses. You quote a static strength. Climbing rope is tested and certified to hold falls, with a limit on the maximum load it will put on the falling climber. It is called "dynamic" because the elasticity is deliberately designed in to provide that maximum load on the climber. If you are going to use the rope for climbing, you need to look at the other parameters of the rope's performance.
Static lines designed for rappelling (and as tag lines and for hauling) are different from static lines for marine use. They are still static (actually they have some minimal stretch), but the wear characteristics are different for the different conditions encountered. But static rappel lines are not intended for use as climbing ropes. If you fall on a static line, you subject yourself and the anchors to a shock load, which has resulted in serious injury in way too many cases. You can rappel on a dynamic climbing rope, and in fact much use of dynamic ropes is the rappel down from a completed climb.
Climbing ropes, both dynamic climbing ropes and static rappel lines, are often subjected to sandy and dirty conditions, especially in much of the Western US. "Dry" ropes are called that because they are intended to minimize absorption of water. But a side effect is that the collection of sand and dirt is also reduced. Not eliminated, of course, but reduced. You can (and should) clean your ropes periodically. On the SuperTopo website, you can find discussions of the best ways to clean ropes.
Anyway, do yourself a favor and buy a new rope which is designed for your intended use. Taking the cheap way out is just asking for trouble. I speak from having started out with a manila rope bought at the local hardware store.