Original iPad, 18 months old, battery failing: replace battery or replace iPad?

Question

At this point in Apple's product life cycle, can I expect the greatest utility by a) replacing the battery in my original iOS 5 iPad,  getting another 2 years use out of an end of life system, b) buying a refurbed iPad 2, c) buying a new iPad 3, or d) limping along until the next-gen iOS 7 iPads arrive, probably in October 2013?
At this time I can't say whether ios7 will run on the iPad 2 or iPad 3. Assuming the iPad 2 has no upgrade path, I don't think that's a viable option. I would expect the iPad 3 will upgrade to its 7, so that's a viable choice, but it's about $ 300 more than the cost of replacing the battery, and it leaves me with an unusable but otherwise perfectly good iPad.
Replacing the battery costs $110 from Ap,ple, or about $30 if I dare to do it myself. This presumably would give me another 2 years of battery life at low cost.
My concern is more with long-term support for iOS 5. I had a wake-up call a few months ago when one of my daily go-to apps, Zite, updated in the App Store to iOS 6 only. CBC's news app also went iOS 6 a few days ago. I still use the iOS 5 version of these apps, but the underlying online services may change, rendering the apps unusable. Both apps still appear as updates in the App Store, each with a note indicating they are iOS 6-only updates. How long until the rest of my favorite apps go iOS 6 only in the App Store?
If I lose the iOS 5 version of an app, either through a crash, OS corruption or accidental deletion, or I haven't properly backed them up (or more likely forgotten where I backed them up!), I'm hosed, because the original iOS 5 apps are no longer available from the App Store.
Gotta end this post here, because my battery's down from 35% to 10% in the 40 minutes or so it took me to write this post. Any suggestions are welcome!

Answer 1

Replace the battery and then sell the iPad.
You may can get the battery replaced at 3rd party repair sources for less $.
iPad Repair & Screen Replacement Services
http://www.ifixyouri.com/16-ipad-repairs
RepairZoom iPad Repair
http://www.repairzoom.com/ipad-repair.html
Mission Repair
http://www.missionrepair.com/Apple_iPad_Repair_Services_s/431.htm
iGadgetResQ
http://www.igadgetresq.com/ipad-repair/
How to Sell Your Old iPad  http://tinyurl.com/85d69lk
Other sources to sell.
eBay Instant Sell http://instantsale.ebay.com/?search=ipad
Sell and Recycle Used Electronics - Gazelle http://www.gazelle.com/
For instant gratification in selling a used iPhone or iPad, Gazelle’s Gadget Trader, an iOS app, is tough to beat. In seconds it detects the device and reveals how much it is worth in good condition. Tap the Sell This Phone to Gazelle button and the deed is done.
Sell Electronics for Cash - Next Worth  http://www.nextworth.com/
Buy My Tronics  http://www.buymytronics.com/
Sell Your iPad http://www.sellyourmac.com/mac-product-guides/ipad.html
Totem http://www.hellototem.com/
 Cheers, Tom