In my opinion, you can go with a small hydronic system. From your measurments, you would need a small boiler, 550 BTU per hour output, 18 feet of baseboard and set it at 55 degrees when not in use. Forced venting should not be a problem as the venting is directed outside. Incase of a power outage, simply open the one zone valve you will need and the system will circulate by gravity. Make sure the baseboard is placed on the outside walls/ The formula for each room size is as follows. Multiply lgth. X width. for room area. Multiply that total X 3000.
Divide that total by the number of BTU per hour output and that will give you the amount of baseboard needed for each enclosure. Example: If you have a 10X10 room, you have 100 Sq. ft of area. Multiply 100x 3000 and you will get 300,00. Divide that by 550 (BTU per hour output), you will get 545.etc. The first three numbers will give you the amount of baseboard, ie. 6 feet to adequately heat that space. With an R factor of R-19 and good windows, you should have no problem. Note: For every 2 ft. in heighth over 10 feet, add 10%.
Hope I helped.
P.S. I've use hydronic systems for years and for me, there is no other option. However, If you have frequent power outages, go to a direct vent system.