All you folks with your B3s and C3s. If you're looking for the real deal, go for an A-100. EXACTLY THE SAME as a B3 or C3, just a different body. Actually, it comes with reverb and internal speakers, making it even better! If you take the sound to the Leslie out of the AO-28 pre-amp (upside down on the A-100) instead of the AO-39 power amp, it'll sound just like a B3. Typically people ask for $1500 for A-100s but you can often find em for much cheaper. I got mine for $700, though I did have to pull everything out to fix and clean some stuff.
Do you need a full 61 key manual? If not, you could go with an M3 or M100 series (the "Baby B3" Spinets!). Same innards as the B3s, just less notes. They have 44 note manuals. The main difference is foldback and manual tapering on the top end. This guy talks a bit about it, he modded the hell out of his M3 - YouTube - ‪Hammond M3 Organ After Manual Tapering and Foldback Mod‬‏
- You can very often find M3s for free if you look hard enough, otherwise you shouldn't pay more than $200 if you're in a city.
Foldback and tapering can be added, although you'll need 48 (or 55) extra contacts for foldback and resistors for tapering. M3s also need several (3, I believe) full length busbars for foldback to work. It'll set you back maybe another $50-$100. Tearing open the organ sounds a bit daunting, but if you know how to solder, it's fairly easy. Just very... very very very... very time consuming. You probably don't want to pay someone else to do it, and they might even tell you they won't do it (for several reasons including time).
I'll say, I very much enjoy my M-111, but playing the A-100 is "that sound". The M series is close, but somethings missing. With a Leslie, that something might not be very much. Check this guy's stock M-111 through a 147 - http://www.youtube.com/watch?v=6lPw0wKu7_M&t=83
As for Leslies, the cheapest you'll probably find would be something like a 25 or 125 (125 is dual speed). These models only have the lower baffle. They don't have the horn, which is a big part of the Leslie sound, but they still sound pretty good. You can hear my M111 with my 25 here
. We used a remote control lightswitch to get the variable speed off the one speed leslie.
If you want a dual-speed, top horn Leslie, the main one people go for is the 122, closely followed by the 147. Either of those will probably set you back around $1000. Sometimes you can find something like a 21, 45, or 47 which aren't dual-speed but have a horn. You can sometimes upgrade them to dual-speed for cheaper than a dual-speed Leslie would cost. There are also some nice sounding solid-state Leslies.