If I sign up for BattleCorp now, can I go back and read the Redemption Rift series?
No. Think of BattleCorps like a magazine - you (only) get whatever gets published during your subscription period, plus unlimited access to the news archive and forums. And - but don't quote me on that - I think you get a certain amount of "current" fiction pieces upon subscription.
As a subscriber you can purchase "back issues", i.e. older BattleCorps publications, through the BattleShop. And just like for all other purchases you get a 5% subscriber discount (even though you need to be a subscriber to purchase back fiction in the first place).
If you're specifically interested in Redemption Rift then you're looking at the costs of a subscription (with 1 month costing the smallest amount of money, 6 months being the cheapest per month, and 3 months somewhere in the middle) plus the costs of purchasing the four individual parts via BattleShop.