What parts do I need to use Di2 wireless shifting on a road bike Also what is the cheapest best way to buy them. I just want everything that is absolutely necessary for wireless shifting so no crank, drive chain breaks etc.
You require these items, all DI2 compatible:


rear derailleur mechanism
di2 controller box (normally strapped to the bottom of your stem)
di2 brifters, or di2 shift buttons that strap to your handlebars, but mean you can keep using your brifters as plain brakes.
battery (one for a wired system, or several for a wireless system)
wiring harness, and secure method of holding wiring on/in the frame so its protected.
wall charger (maybe USB based, maybe a separate wall charger)


You probably require 


front derailleur mech unless you want to make it a 1x bike


For your testing, you really will need a road bike too.  The derailleurs will try to calibrate themselves and may go into fault mode if there's not a cassette or chainring detected.   So that means a modern cassette with 11 gears, maybe 10 (I'm unsure on this point)

Generally speaking, the whole groupset will cost about the same as a bike with that groupset fitted.  Used bike gear drops in price very fast too, so don't think you'll get your money back by selling it on after the testing, even if its never been ridden.

An alternative suggestion might be to make contact with the Shimano distributor in your area and see if you can get access to test/demo kit for your purposes.  As long as you're not cutting out a reseller, they might be happy to talk.  Or involve a LBS