A good place to start would be a website like hogcult.com, which lists a bunch of knives with their names, and you can start to get familiar with them.
To get started, you can troll the "for sale" threads in the forums, although if you're not familiar with the models you'll have a tougher time deciding what is and isn't a good deal.
In many senses, your best bet, price-wise, is going to be to get a Busse directly from the factory. You're in luck, as there is a real beast of a 6" blade (Tank Buster) available right from Ohio in the store at
http://www.bussecombat.com
If you want something a bit smaller, the Company Store (limited runs available for immediate shipping, rather than made-to order custom runs like the main website) has the Sus Scrofa.
http://www.bussecompanystore.com
As far as the second hand market goes, the various steel heart, natural outlaw, and mistress models seem to be big favorites for large knives with a whole lot of power. Figuring out just which version you want, out of the eleventy different variants, might be a little bit trickier
Edit: As far as how I personally started - I pre-ordered a Scrapper 5 from Busse's sister company, Scrap Yard, and while I was waiting for those to be built and shipped (two months or so), I bought a knife or two on the second hand market, and then I went to the BLADE show in Atlanta and bought a whole pile of knives. Since then I've done some selling, trading, and more buying, and have spent more money on knives than I have on mortgage payments
