I'd get ahold of that guy and ask him what units actually get 84% because there is only one brand that makes a 90+ direct vent wall heater that I'm aware of and that's Empire and they don't sell a small one. It is 17500 input on high stage(15778BTUhr output) and 8750 on low stage.
Empire Comfort Systems PVS18 17,500 BTU UltraSaver90 Plus Vented Wall Furnace
$2k for one. The issue is that a 2600sq ft house isn't going to only have one.
The brand of these that I like is Rinnai
Direct Vent Wall Furnace | Rinnai
Here's everything 81% AFUE or better in the ahridirectory.org Direct Heating section that runs on propane.
Certified Ref # Model Status Trade/Brand Name Manufacturer Model Heating Capacity (Btuh) Input AFUE Equipment Type Fuel Type
7434751 Active EMPIRE EMPIRE COMFORT SYSTEMS, INC. PVS35 32130 35000 91.8 Wall Furnace Natural Gas,Propane Gas
7434724 Active EMPIRE EMPIRE COMFORT SYSTEMS, INC. PVS18 15778 17500 90.2 Wall Furnace Natural Gas,Propane Gas
2029759 Active EX08C RINNAI AMERICA CORPORATION EX08C (RHFE-202FTA)(P) 6642 8000 83.0 Wall Furnace Propane Gas
2029767 Active EX11C RINNAI AMERICA CORPORATION EX11C (RHFE-265FTA)(P) 9000 11000 82.0 Wall Furnace Propane Gas
2029769 Active EX17C RINNAI AMERICA CORPORATION EX17C (RHFE-434FTA)(P) 13700 17000 82.0 Wall Furnace Propane Gas
2029771 Active EX22C RINNAI AMERICA CORPORATION EX22C (RHFE-559FTA)(P) 16898 22000 82.0 Wall Furnace Propane Gas
2029809 Active ENERGY SAVER RINNAI AMERICA CORPORATION RHFE-1004RFA-P 29200 36500 82.0 Wall Furnace Propane Gas
4861399 Active ES38 RINNAI AMERICA CORPORATION RHFE1004FTA 29200 36500 81.4 Wall Furnace Propane Gas