Welcome to the yard Mick. You are definitely in the right place for tough work horse knives. Scrap Yard has one of the best price to performance ratios you'll find.

For the most part, the only real difference between blade coatings is their color. However, some coatings will be different from others - one may be more durable, or smoother, meaning it would tend to get less gunk stuck to it if cutting sappy wood or something.

For Double Cut, it's not actually a coating at all - the blade is bead blasted with two different sizes/types of media - hence double cut, cut once with one media, then again with another. It is definitely rougher and less reflective than a satin finish.

As far as purchasing, that's always the toughest part - since the knives are released in limited time runs, it's sometimes hard to get your hands on older models. Your best bet is to watch the Busse/Swamp Rat/Scrapyard Knives for Sale board at bladeforums.com. OR you could buy the scrapper 5 LE, which is available right now from Scrap Yard, at factory prices (most Scrap Yard knives sell for more than original price on the second hand market)