I’m actually not super concerned about the shoulder season low and slow, my heat pump is pretty efficient. I’m more just needing something I can “set it and forget it” for at least 12 hours in the winter months. My current stove I have to load every 6 hours and every reload involves 45 mins of fiddling. If the temp drops in my stove room(finished basement) then the whole house temp drops and it’s a pain to catch back up. So my motivation is really more the big firebox and thermostat. Not to mention I’m currently shoveling ash every four days.I almost feel like a 27' vertical could offset that ugliness at the beginning.
On the other hand, BKVP is the one who gets all the "your stove doesn't work" phone calls from people who didn't read the manual, so he has more experience than anyone with this particular situation!
I personally wouldn't expect any issues heating the house in the dead of winter, but one of BK's biggest draws is the black box mode for shoulder season, and that's the part more likely to suffer in warm weather as draft decreases and you start running the stove lower and lower.
I'd try it anyway if the price was right! Maybe you get a stove that's less capable than a properly installed BK, but just as good as a tube stove. And you can always choose to upgrade the venting down the line.
And at the worst, you end up unhappy with the stove. It sounds like you could resell for more than you paid anyway, as you mentioned the price is "too good to be true". 👍