Hmmm. The trouble with most solo tents is that they're designed to be super lightweight. And that means they have a lot of mesh in the canopy, which makes them pretty chilly in the winter. Typical is the MSR Hubba 1P tent ($250), which is reasonably roomy for a solo shelter and really easy to pack at just about three pounds even. The fly offers excellent coverage, but the canopy is breezy.
So, probably the best choice is something like Black Diamond's Hilight ($370). It's a small two-person tent—great for one person—that is designed to be a four-season/mountaineering tent. It's very rugged, and extremely weather-tight. It has a single-wall design, and uses a proprietary waterproof-breathable fabric. Weight is just over three pounds to you are not paying a penalty there. The one drawback is the lack of a vestibule. You can add one, but that adds a pound and $135. Still, it's what I'd probably go for.
As for a bag, I should think a bag rated to zero degrees F would be the minimum, and one rated to -10 probably even better. A really good choice: Western Mountaineering's Lynx MF ($585). Rated to -10, it's a very light bag (3 pounds 2 ounces) that also packs down very compactly. Impeccable construction, with a light but tough microfiber shell that I think is nearly ideal for a cold-weather bag. I know it's not cheap.
Marmot's Lithium ($460) might work for you. It's rated to 0 degrees F, is a very well-made bag, and uses high-quality 850-plus fill down. It has a nylon ripstop shell, and polyester lining. Plus a snug-fitting hood. Very toasty bag. Like the Western Mountaineering bag, it will pack down very tightly. So you can easily cram it and the rest of your gear into a pack—but you may have to check it...
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