Older ARC preamps (such as a SP3A-1) and power amps (such as a D-70MkII) can be had for roughtly $800 each (or possibly less) in good condition. They are excellent pieces of equipment even by today's standards for those prices. For less $ you could look at pretty much anything in the Dynaco line. Dynaco made a number of amps and preamps that you might consider; they also made some tube CD players (such as the CDV2) that occassionally can be found for about $500 or less which will pretty much blow away anything else you can find for the price. Beyond ARC and Dynaco, other less expensive vintage equipment is available - just search on eBay under "tube amplifier", etc to get a sense of what's out there. Once you home in on candidates, Agon and AudioAsylum should provide lots of input on specific models of interest. As with anything, there is a point at which the price will be attractive but the gear won't meet your expectations, so make sure you are as comfortable with the seller as you are enthusiastic about the piece of equipment.
A tube-based system is worth pursing - hifi without tubes wouldn't be quite as satisfying (just my opinion).
Also, (as you probably know) it's generally advisable to pick your speakers first, then find an amp that is suitable to drive the speakers, and then pick the preamp and CD or other components in a way that adds synergy. Tube equipment is somewhat different than solid state, but it's still part a system that needs to work together in your room.
It can be a blast and rewarding, or frustrating (if you are into it long enough, it's likely to be some of all that); do your homework, try to listen to the models you are interested (sometimes you can, but sometimes you have to go on recommendations), trust your instincts, and remember - it's often a journey rather than a destination.
