As Bill said, Martin & Taylor are the first names that come to most people; the best way I've heard the difference described is that with Martin you hear the wood more & with Taylor you hear the strings more.
And Gregory's right, mid-'80s D-18s are not rare, and you should be able to find a decent one well within your budget. But please, don't let him limit his search. Yes, play M & T, but don't forget Gibson & Guild, as well. Santa Cruz makes marvelous instruments, but you'd need used to come in under $3K. Larrivée is a little further down the price scale, but I've played a goodly number of them - dread, 000, jumbo, parlor, 6-string, 12-string, rosewood, mahogany - and every one's been absolutely magical!
While we all want beat the lowlife who caused this dilemma about the face with a Teisco Del Rey, let the search be an adventure. And I wouldn't only play dreads, either; there are great sounds in other shapes.
But tell me you're buying me an acoustic of my choice and I'll probably go with a Martin M-36; tonal balance of a 000 with most of the volume of a D. (Also acceptable but harder to find: M-38, M-30 Jorma Kaukoken. M-42 David Bromberg)