A lot of people ferment 5 gal batches in 6 gallon better bottles! The trick is you may need to run a blowoff tube (which you accounted for in your list) if you're running a beer that goes wild in fementation. I am 1 week into fermenting my 1st batch in my new 6 gal BB and it's been doing great. No blowoff tube needed, just the airlock. Granted, bigger beers may make quite a mess if you push it.
See this post about the differences if you haven't read yet.
I was very similar to you in putting my equipment together. Decided I was going to buy things separately so I wasn't paying for stuff I didn't want in a kit. Also, you don't necessarily need all of it on brew day (ie, capper, siphon). Some other ways I kept SWMBO happy by cheaping out:
*Food grade buckets at Lowes + $3 spigot from LHBS = half the price of buying a "bottling bucket."
*I am borrowing an auto siphon, bottle wand and capper. I will buy my own soon, but for the 3 hours I'll need them my friend didn't mind.
So, for my initial investment, i spent a whopping total of $87, and that included the ingredients for my first batch.