Not sure I understand your goal here....
You want OS 3.1 rather than anything later, which is only advantageous for games, but you're expecting to need a PPC accelerator.
If you want to use your PPC correctly you should at least get OS 3.9 but you'd be much better off getting OS 4. Forget about DVDs though, you'll never be able to play them well on a classic Amiga, no matter what hardware you throw at it.
For the above tasks though I'd say it's a no brainer - you have to have OS 4, which'll do everything apart from DVDs. You won't regret it (I didn't).
Personally though I'd just get a SAM and keep a 1200 just for games.
