You may or may not find this link of interest.Chubby Data Logging
It was my attempt to record temperatures as measured on the stove and on the flu pipe as well as refills and shakedowns.
BARO - regardless of what anyone tells you - "you may or may not require a baro".
And even if you require a baro...it doesn't mean you absolutely need one.
You should be able to control the Chubby with the manual pipe damper (built into the flu outlet of the Chubby) and the draft air intake slider.
To be honest I think it's safer to not use a baro then it is to use one.
This is just my opinion - I am not a professional stove installer.
There's a ton of people on both sides on this barometric damper topic...you could research it until the cows come home.
I do not use one.
This a short clip of what the baro does - it may help you understand the operation. They use these things for oil furnaces also.
Most of the units I've seen can be flipped around or the counterweight adjusted so you should be able to make do with any of them, in either a horizontal or vertical installation.https://www.youtube.com/watch?v=WBV66Q9XKNc
Also - I'd hate to see you go with a refurb and shell out large sums of money. My first impressions of you were that of trying to economically heat your house - in such a way you could back up to the thing and roast your behind. You certainly do not need to buy refurb'd. Honestly there's not much to these stoves. If the grate is intact it may bode well for the entire stove. You should not have to spend more than $350 for a perfectly good unit - perhaps not in the middle of January...but in June/July you may find prices drop to more reasonable levels.
And no - you cannot go from 6" to 5" flu vent.
It is both illegal and unwise.
You can always go bigger - for example a 6" stove outlet to 7" or 8" etc....but never the other way around.
I hope all this helps. I'm not trying to be absolute here...but I do subscribe to the best bang for the buck theory.