I would say even with a cracked heat exchanger you would get warm air . Just contaminated with Co. If the air going in is cold the air coming out will be warmer but may not feel hot. It also depends on how the ductwork if sized and run ( thru unheated space).. uninsulated?
I would say not flame roll out otherwise the soot would be up the front of the burner box. Also there is a temp sensor "roll out switch'' located just above and behinde the burner designed to shut off the gas in the event that flame or excessive heat were to go into that area.(flame roll out) ...
What kind of pressure do you have going in and out of the gas valve? A cracked heat exchanger will only effect the flame when the fan has come on (blowing air on the flame)if it is not burning correctly with the fan off then you have another problem.
When you converted it did you change the valve pressure? Should be set around 10" wc for propane. If so check to see that you are getting at least 14" wc from the tank.
Before you buy the property. I would check to see what zoning restrictions you may encounter. Be sure you can build it big. If you are putting in a lift I would go an extra foot or two and put in a 14' tall door, then you can get anything in there (RV ,Truck, whatever).
That is the way most Mobile Home units are installed. It is a down flow unit with the "closet it is in being the return air duct. It must have a vented door.You could add a return plenum with a filter rack or just close the door and not look at it and go work on something in that nice barn.....
You may not always get a CO alarm. Most CO meters are a cumulative type and need high levels for an extended period of time. All combustion creates CO but only high levels are seen with incomplete combustion. Clean blue flame like your gas cook tops do not produce the high levels needed to set...