I recently did a huge rehab on a 1600 sf 3/2 SFR that was post and beam construction, which made it very difficult/cost prohibitive to run ductwork in an attractive manner. Also chimney was bad. Long story short, I installed a total of 5 direct-vent heaters throughout the house, and they are awesome. It allows zoned heating and is quite efficient.
2 caveats, though. First, some thought needs to go into placement and air flow to make sure the home is heated thoroughly. Second, when I had the house appraised for a cash-out refi, the appraiser was not familiar with the heat and needed some more info. Then, underwriter had questions as well. In the end, I was able to supply the info and everything went off without another hitch, but people are naturally cautious with the unfamiliar. The problem is with the term "space heater", which makes people think of units which are specifically NOT designed or legal for primary heating source.
I would have a heating expert familiar with these units inspect - if he is confident that they are adequate and appropriate, then proceed. I am entering the second winter with these units in the rental and tenant has been very happy.