OK, assuming you can't get a gun (like the prior poster, I'd suggest a shotgun; you can get a Remington 870 for cheap -- less than $300) and you need a knife, I'd go with the biggest fixed blade you can get. If you're not into knife collection and just need it only for HD, it can be inexpensive.
A large bowie, a khukuri or bolo would do the job. Myself, I have an Ontario Bagwell Hell's Belle bowie (16.75" total length), HI Kobra khukuri (19" but I'll be buying a 25" other kind of khukuri soon) and assorted other long khukuris. However, those are relatively pricier than what you need. (The Bagwell retails at $350 although you can get it on the net for $200, and the HI khukuri goes over $100.)
You can get a solid khukuri with a 15" blade from Khukuri House for less than $100 easy, or one of the less expensive offerings from Cold Steel. Ontario has cheaper long bowies that are affordable.
For HD, no need to fool around with a small folder. A folder is an option for SD away from home where concealment and portability are the priority. For HD, you want range and quick man-stopping capability. One swipe of a long, heavy khukuri can end a fight -- and it doesn't have to be too well-placed.