What you should do, right now, is get your rank up to 8-1 (I think. Been a while since I ranked one of my alts up).

All of those items will be available from the supply depot (Except the volt line and Avenger Line) for 3500 energy.
You can also upgrade items, so it's useless to purchase both a voltedge AND a voltbrand.

Your 4* costs are acceptable, for 2.5kE.

Most clean 5* items go for around 5-6kE, depending on if it's a boss item. Price spectrum is like Cobalt Line -> Shadow Lair items.
I've seen Snarbolax pieces go for under 6kE.

Each item has an unbind cost, so you won't find anyone willing to sell any 5* equip for under 4kE.
The additional cost is for the effort required to level an item up.

There's no point purchasing these items if you're not high enough rank - you won't be able to equip them anyways.
So start doing missions instead of wasting CE on items that you can't use, and are redundant.