Wow....here we go. First off, you will not regret building a beer budget EV. No matter what anyone says, GO FOR IT! It's a lot of fun.
In your "Things I know I'll need" list you can do without the DC-DC converter. For any converter with the output you require (I say 40A mininum @ 13.8V), you will pay hundreds of dollars. I went with a group 31 truck battery (free) and a $20 PWM trickle charger (puts out 2.2A).
Battery cables can also be free. Make friends with a commercial truck salvage yard. They all use 4/0 cable. With a low budget EV you are likely going to be at 72V, so amps will be a bit higher than normal. Big cable is good for higher amps.
My Alltrax AXE7245 (72V/450A) controller was $300 used on eBay. It has been absolutely stellar. I have never experienced thermal cutback in my 3000 lb lead sled, and that says a lot.
Go with a forklift motor, if you can find one. Here is an EXCELLENT guide:
Using a forklift motor, and choosing a good one - DIY Electric Car Forums
MAKE SURE IT'S A SERIES WOUND MOTOR!
Don't forget:
-A bunch of 1"X1" angle iron, 1"X1" and 1/2"X1/2" square tubing for battery racks.
-40-50 4/0 copper cable ends
-Various nuts and bolts
-Chunk of 1/2" aluminum for adapter plate
-A piece of 5/8" particle board from the Ikea "as is" section for a control system mount. (This is my top secret item....shhhhh..don't tell anyone). You can mount ANYTHING to it (controller, contacter, etc with a couple of particle board screws.
-Lots of butt connectors, ring terminals, heat shrink, 18 gauge wire.
-Potbox. $60 for the PB-6 @ Cloudelectric.com.
-vacuum pump. A used Volvo pump from a junk yard/eBay + a $25 vac switch.
I will probably think of more.....
Good luck!