IMO I would save up and just assemble piece by piece.
I've not done a Vader but would like to some day, but from what I've learned so far with Fett, doing things right the FIRST time will save you a bunch of coin in the long run.
My advice to you is contact local costume rental places around you and see if they have a Rubies Vader. If they do go check it out and ask yourself if you'd be happy with that kind of quality or not. That being said, bring a pic of Vader with you and compare the components piece by piece. When you show up at a costume party as an inaccurate Vader it's awesome, but when you're trooping and get next to an accurate Vader it won't feel as awesome.
This is just my $.02!