Well.... This is getting pretty vast but I will try to help
Let me first say that most btu per square foot ratings are grossly over-estimated. Now in regards to but output, yes there are many furnaces that can produce over 125,000 btu. However those ratings are Max ratings, which essentially means there nearly a useless number. So yes stove a will produce let's say 200kbtu per hour but only if your putting wood in it every 2-3 hours. Most homes under normal winter circumstances will stay warm from 35-55k btu per hour. Further north ya go the more that number will rise but generally speaking its a good place to start. So, as far as btu ratings what's important is how many btu per hour will this stove produce over 12 hours or more. This is where an experienced dealer could take a lot of the guess work out and keep you from racking your brain over information that is sketchy at best.
The same thing goes for furnace coils, another joke in my opinion, folks try to size them based on there btu they think they need. Well first of all the company making the coil has no idea how hot your water is, or how fast the flow of the water is, so once again those numbers aren't reliable.
Now as far as ridgewood vs a portage and main, portage and main is one of the highest priced units, yes they are good, I'd like to sell them here but from my market research I don't think I could sell many based of price. But, in terms for you, ml 36 would likely run you over 8000 for the unit alone. The finish and look of the stove are superior and probably the biggest thing is your buying a brand, also with larger companies there is a greater chance they'll be there down the road to support you. The earth and the ridgewood are both good units, however e design is simpler and somewhat less efficient, were not talking half as much wood as an 8000 dollar stove but let's say 15% more. Just a rough estimate, because being involved with testing of boilers in general the numbers there are a huge misconception as well. Also look at differences in warranties and see which one appeals to you, and then perhaps do enough research or talk to dealers who sell for them and others to see which company they represent stands behind there product the best, if the dealer only sells one brand, your answer may not hold a lot of water haha
I would also warn against the side arm, they allow the water to equal stove temp, and then were relying on a mixing valve to cool the water down before you use it, if the mixing valve sticks, well it's gonna be 180 heading your way. A 20 plate exchanger can make endless amounts of hot water without relying on the convection cycle that the sidearm does, therefore the water can only get to perhaps 135, so there it makes for a safer and simpler install. I dont like the feeling of liability when dealing with sidearms especially when a child could potentially get burned.
Sooooo, yea there is a lot to it, and good information is hard to find.
Hope this helps...
P.s
Also in regards to why the dramatic price increase from let's say a earth to a central? You'd be shocked to find out how much per stove the big companies allow for marketing.