Questions (very belated) re Parashat Chayei Sarai

Here's the text in question (or the text that's raising questions):  Genesis, chapter 25.

The text says that Avraham (Abraham) took a wife, bore six sons with her, gave all that he owned to Yitzchak (Isaac), then gave gifts to the sons of the concubines and sent them away to the east.

First of all, why would any man in his right mind father six children when he's over the age of 100 (or however old that is in contemporary terms)?  How and/or who does he expect to take care of them?

Second, how many concubines did Avraham have?!  Hagar was a concubine, but Keturah is clearly called a wife.  

Third, does this mean that Avraham *finally* got around to giving a decent gift to Yishmael (Ishmael), whom he had sent out to the wilderness as a teenager with nothing but a container of water and some bread (a death sentence)?  "Gee, thanks, Dad.  Now that I've spent decades paying off my medical-school loans and putting my kids through college, *now* you decide that I could use some financial support?"

Who wrote this chapter?  What the heck was/were they thinking, or weren't they thinking at all?  It makes no sense.