So this is a very complicated question. My first question would be: are you replacing a boiler or installing a new hot water system? If you’re replacing a boiler, I would replace it with something extremely similar or identical. The installer of your system picked that boiler because the BTUs were appropriate and it included the appopriate gas or electrical hook-ups and the appropriate pump for your system. As far as brands go, I really can’t help. I forget the brand I used, but it’s got a warranty and it’s the appropriate boiler for my house, so I figure it doesn’t particularly matter.
My advice is to purchase a boiler online and to pay a plumber about $1000 to install it. If you do that, you’ll save about 50%. The biggest problem I had was when they dropped it off on a pallet in front of my house. But if you’re willing to do a little work and find a plumber who’s willing to do the install, you can get away with paying about $3000 instead of the $5500 or so a Stevens might ask.