For benders and dies:
If you like cages or bumpers, you'll probably want a 1 3/4" die in the 6.5" to 7" radius range. That would be my first die set. The second would likely be 1 1/2" tube in the smallest radius offered. With these two you can build just about anything.
As for a bender, you have to decide how easy you want to make things on yourself. The JD benders and Pro Tools 105 bender are great benders and work very well. I've had both over the years and they work about the same (less gadget differences). Upgrading them to air over hydraulic is possible and doing so will result in a very nice bender and fairly repeatable bends. The downside to this type of bender (manual or air over hydro) is that you must rest the die pin to bend angles larger than ~25% (that's a guess honestly). This can result in repeatability issues (making the same bend twice) as the material can slip, rotate, or basically move when resetting the pin. The upside is that you can bend greater than 180* if you need to.
If you plan on getting a air over hydraulic setup the Pro Tools HB 302 (aka One Shot) is an excellent choice and ends up being less expensive when compared to the other style benders on air/hydro.
Shown above and found at
http://www.pro-tools.com/302.htm, you can start your bend and go up to 110* in a single shot - thus the name one shot. No slippage, no rotation, and no moving because the pin does not need to be rest. they are simple machines and have less wear points when compared to the others. The way the bender operates makes it easier to support the material and start your bend as well.
There are other companies that sell similar machines and it may be worth your time to check out the following sites to compare types of machines and prices:
http://www.pro-tools.com
http://www.tricktools.com
Regarding plasma cutters...different than a welder. You want to buy the biggest one you can afford as the bigger the machine, the faster and cleaner your cut will be. A little 110V machine will have to hack through 1/8" and a (220 amp example) Powermax 1000 can cut 1/8" about as fast as you can move your torch. Anyone who has used both can tell you the difference.
Basic other fab tools will be required. I can't live without my 4.5" angle grinder. A chop saw can work but a small bandsaw works better (less sparks, noise, and health issues). A torch is a good idea as you'll always have to cut something off (if you don't have a plasma). A drill hand drill can work with a vise but a drill press is much better and safer.
Good luck. Years ago I started with a chop saw, a torch, a 110v welder, and a 4.5" angle grinder. You have to start somewhere.