So am I correct in thinking that since DC has 'rebooted" several times, there are several complete origin stories for both Batman and Superman? Are any of them in continuity with each other and contain both an origin and death/retirement story?
Comics are not as straight forward as that. (Though I think it would serve them well to try such a strategy, but that's wishing after impossible things.) There are numerous tellings of origins, continuity is loose at best throughout DC's various histories, and nothing contains both origin and death/ retirement - for one thing, these characters never die or retire for good.
For Batman, try this run:
Year One - absolutely the place to start.
Strange Apparitions - a trade Paperback collecting a run from the 1970s that is among the best of Silver Age tales.
Early arcs from The Legends of the Dark Knight, as mentioned above -
Prey is very good,
Shaman's okay. The early Tim Sale /Jeph Loeb stuff was golden, especially their Halloween specials, which are collected in a TPB, as well as
Blades. Faces by Matt Wagner is superior too. These are material from the early 90s, following the revisions of Batman that came from Miller and the first Burton film. A great one-shot from this era is
Storm.
If you can get your hands on
Dark Knight, Dark City, also from the 90s - a four-issue arc under the Batman title, it's one of the best Batman stories ever told.
Personally I thought Loeb's writing went off the rails with later stuff such as
The Long Halloween and
Dark Victory, but they are well-loved by others and very atmospheric, so if you don't mind a rather nonsensical plot and a Bruce Wayne who is kind of a zombie - go for them.
For stuff more in continuity now, the
Cataclysm/ No Man's Land TPBs (there are 5 or 6 in all) are really quite good. More Batman family than Batman though, with lots of plot lines involving Nigthwing, Oracle, Robin, Batgirl and Huntress. Just about all the villains make appearances at one point or another.
Other not to be missed material is
The Killing Joke and
Arkham Asylum.
After you've checked out all this - then read
Dark Knight Returns for a very good retirment tale.
Corollary to Batman, but some of the best comics material ever put out by Dc, is
Gotham Central. HIGHLY recommended - it tells stories of Gotham cops, and is entirely brilliant. I got the whole series from the library, and you should definitely check for a lot of these books there if you don't want to invest. They'll have many of the titles mentioned.
I'm less familiar with Superman, but a couple things to be sure to read are
Birthright, a lovely retelling of the origin and early years. And, while I think it's a tad overrated,
Whatever Happened to the Man of Tomorrow is a retirement tale attached to the Silver Age. Another good TPB is
No Limits, which is a kind of restart after a long, long arc that took things far from the basic Superman set up. It's after Lois and Clark get married so is a nice introduction to the new era of Superman.
Throw in
JLA: New World Order, Tower of Babel and
League of One first to get a sense of the larger universe and the heroes working in teams.
When done with all this, read
Kingdom Come.