This is an old thread but for those that might look to it in the future I'll give some input. I've spent a fair bit of time sleeping out in winter including sleeping at -40F with no tent and have several bags including a Stephenson Warmlight that keeps me toasty at -40F.  Those looking to camp out in winter have a lot to consider if they have not worked there way to winter camping. By the time you want a -30F bag it would be best to have a fair bit of experience such that you don't really need too much advice other than this bag maker vs that, but such isn't always the case. One consideration is whether you expect to spend multiple nights out. Multiple winter nights require either a vapor barrier liner of some sort for a down bag or a synthetic fill. For winter bike packing, bags are often for emergency use and most folks would only expect to spend one, two or three nights at the most, which would allow down to be used, with the user taking care to air out the bag in the sun during the day if at all possible.  Goretex shells are super great if going without a tent but make a bag much less compressible. IMHO for winter bikepacking only down makes sense.  If you aren't going way the hell back and have a SPOT for emergency or can otherwise think that the bag would only be used one night or two at the most you don't have to worry about a vapor barrier. Get a bag without it and buy a vapor barrier liner if/when that need arises. All the top names make good bags. Avoid duck down and go with a well known name you can find on sale and you're good to go. Some advantage of down is lost in the need to have a thicker pad than when using synthetic so there is that but overall for -30F, sythetic and bikes seems to me, kinda cray cray.

A rental would be the way to go after flying into Fairbanks and getting situated.  It's very unlikely you'll run into -40F in March but you'll want to leave enough time for the excursion that you can delay your planned trip by a few days because if you arrive on Day 1 and expect to ride on day 2 or three you may well find a snow dump happen on day 2 or 3 and if the whites get a big dump of snow just as you hope to do the ride you'll be out of luck. In March you have option to see the world ice carving championships as well as several dog mushing races depending on the weekend. Your very best (but expensive) option is to wait will just a day or two before your planned time and buy your tickets at the very last minute when you know the weather prediction looks decent. If you buy tickest a month ahead of time it could all be for naught if the weather turns horrible. You won't be riding on a foot of fresh snow...you need at least some trail to ride on that's been packed by snowmachines. I could be corrected by people much tougher than I am but for most folks up here, we ride on trails that are packed by snowmachines or dog sleds and that doesn't take long after a dump. Batteries, lights and layering are critical.

The best I can tell you is that regardless of your weight, the fatter the tire you can put on and the lighter the bike you get, the more conditions you will be able to ride in. At your weight you won't need 5" tires often and maybe never depending on your conditions but I'd bet good money that at some point you'll find your 3" tires inadequate and at some point you'd find your 4" tires inadequate.  One big advantage of a bike capable of 5" tires is you can use whatever tire you want...and if you choose you can also build up a set of "plus" tires. I built up a set of 2.4" tires for my Fatback Corvus this summer that allowed me to use it on some dirt mountain rides.

I'll have to disagree with Smithhammer.   I'm not sure how many tents I've owned over the years but quite a few including winter, 3season, summer tarps, etc.  I've thrown my sleeping bag out on the snow without a tent at -40F and been stuck in my Himalayan Hotel for the better part of a week.  I agree that things like snow load and issues like windblown snow and even snow flaps and such make winter tents and winter mountaineering tents special but the ability to shut the tent up tightly and light a hanging candle is huge. While the sleeping bag is great for sleeping, if you can zip up the tent and have a small bit of ventilation and a candle going you will be able to take off layers and layers and sit around very comfortably in a manner you could not in a three season tent.  A three season tent WILL work but you'll just be much, much colder and you will want to check weather reports before doing winter camping in a three season tent (or I would) because wind and snow could be an issue. I agree with Smithhammer in that the snow  load / wind load is a more critical issue. A candle inside for warmth is a comfort issue, a collapsed tent is a safety issue.  Good you're learning...