Open up the side door and the front door and take a picture. The back plate with the date on it is the fireback, if its a single piece and warped or cracked, its not worth much as the rebuild kit is more than its worth. if the there is a seam from left to right on the fireback, its been converted to the two piece back. The two piece backs can also warp but its less money to repair. it likely its needs resealing. There should be a steel certification plate on the back. It has the short legs so it was designed to sit on a hearth. Unrestored but with a good fireback a swag at value is around $300 to $500 dollars. They were good stoves at the time but oversized for most modern homes. They really need to be hooked up to 8" flue. I have one in my basement.
The thermostat cover on the back can also be a clue for date. The backplate date is the casting mold date, not the mfg. date, but it will provide an approximation. Solid doors make this a Defiant I, II or III.
Model #: 0019 & 0028 Defiant I and IA.
Manufactured from 1975 through 6/1980, Defiant I has a one-piece fireback; Defiant 1A is a Defiant I converted to a two piece fireback, sides wrap around the front; Defiant IAI is an I or IA converted to a two piece fireback and two-piece baffle, sides wrap around the front. All models have oval griddle with no top loading.
Model #: 0019 & 0028 Defiant II
Manufactured from 6/80 through 12/84. Has oval griddle with no top loading, sides wrap around the front and two piece, concave shaped fireback.
This is as close as I could get without more information about the fireback or more pictures. No idea on price. Personally, I wouldn't even take it off your hands unless it was the only source of heat I could find. No offense, its just old and inefficient at this point
planeman and begreen are on the right track. This is a Defiant II as it has the front that wraps around the sides, and has the fixed oval griddle (no top loading). Somewhere between 1980 and 1984. Peakbagger is on point too, because if the fireback is trash the entire stove may be worth more as scrap.
If you could find that 4 digit mfg date code, the breakdown is DDDY, and since we know the decade (1980's) a code for example of 1973 would mean the 173rd day of 1983