You just need a thick insulated jacket that’ll trap as much warm air as possible with a windproof outer to prevent it from being blown away. It’s not rocket science, insulation is just trapped air, warmed by your body. The reason the Buffalo isn’t enough is that it doesn’t trap that much air. It’s simply not thick enough. Montane used to do an insanely thick version called, erm, the Super Eight, I think, but it was so bulky you could barely move your arms in it, anyway…
You also need a proper warm hat to minimise heat loss from your head and ideally warm legwear too. If you don’t insulate your legs and head, you’ll get warm no matter how toasty your jacket.
I would’t waste money on down, its main advantage is warmth to weight ratio and packability, but for just sitting about that doesn’t matter. I’d just get something synthetic, with a water-resistant outer. I wouldn’t buy an Altus, it’s quite lightweight as these things go. Have a look at the Keela synthetic stuff for shapeless, but affordable and effective insulation maybe. Or one of the bigger Snugpak jackets if you can find one cheap.
If you’re minted, just buy the warmest down-filled jacket out there, but expect bankruptcy as a side effect…