Um, you're mostly on-point there. Some quick things to review:
Early backwards-compatible PS3's seem to have scaled down in value to reflect the overall MSRP of PS3s in general, but backwards compatible models still have a decent value proposition. I wouldn't call them "cheap". Early non-backwards-compatible PS3s are likely to be less expensive than new slim models, but if you're going a non-backwards-compatible route I'd recommend a newer slim for the sake of reliability.
Here's a short list of PS3s that you can play disc-based PS2 games on:
- 20GB Launch Model (hardware based PS2 emulation, no Wi-Fi)
- 60GB Launch Model (hardware based PS2 emulation, Wi-Fi)
- 80GB 1st hardware revision (software based PS2 emulation, there are a few flavors of this model)
All PS3s play disc-based PS1 games.
Launch model and early-gen "fat" PS3s can be prone to thermal breakdown (the yellow light of death), so if you buy a used "fat" PS3 keep that in mind. It's not as widespread as the hardware flaws found in early gen 360s, but it's a noted hardware issue. Sony does repairs on all makes/models for around $100/$150.
A PLUS membership is $20 for 3 months or $50 for a whole year. You get access to free games on a weekly basis, including a lot of PS1 games and many full-fledged games like Infamous, Little Big Planet, Ratchet & Clank, etc. discounts on games that non-members don't get, and early access to betas not open to the general public. As long as you keep your membership you can play anything you get for free. If you cancel your membership you can't play the games you got for free but you can play the stuff you bought on discount or non-game items given away for free (DLC packs, add-ons, avatars, etc.).
http://us.playstation.com/psn/playstation-plus/
FYI as of now you get access to that Ratchet & Clank game you liked when you join PLUS in addition to 11 other games. Check them out on that link.
As Kupo indicated - that deal ends on Tuesday of next month.