The difference between stable and dev is very straight-forward; stable is more stable, dev is actively being worked on (in other words, in development; expect bugs, expect issues). Stable is typically older than the other updates, but is also usually more reliable in terms of being in a stable state.
Also, for that other question, using older saves can be tricky. There has been a recent overhaul/change on things, preventing most older saves from working unfortunately. I often try to work around it by getting whatever save I want to continue at the moment before death, and just dying and respawning while inheriting everything. This can sometimes work. For the most recent changes, however, I don't believe even that will work.
*Update mid-reply. I decided to test it for you. Using a 10.7.5 save, it doesn't actually even let you try run it - so you will definitely need to start new. (or cheat things in) I don't think someone could fix it for you... but I wish you the best of luck. I'll update this reply if I make any progress in getting it to work.
+Edit 1: just grammar
+Edit 2: the stable version is 10.7.5, just so you know. I would recommend playing the Latest version - which is at B.0.10.8.22
+Edit 3: Answering your first question... Honestly, I am not perfectly sure what the options other than $30 gets you - but that one gives you everything, so I recommend that c:
Going off context clues, RedTrainer has mentioned that the $2 option has the stable version, and on Clusters, it says the option after that comes with dev build. The one after that includes 'art source access' and the $30 one is just everything. Hope that helps!
Edit 4: Finicky grammar