-
Speedheater Cobra is the cheapest infra red stripper. It's not really designed for doors but it does the job well enough that it's just boring not boring and painful. It's also not cheap but there should be a healthy second hand market.
Otherwise Metabo make a machine called the LF724s. It's a proper handful, probably the most dangerous tool I own. Has a habit of eating it's own safety guards. Basically an angle grinder with tungsten blades attached to a wheel. It ejects tiny sharp bits of paint at around 1000mph, they are all aimed at your eyes. Anything it touches is reduced to fragments. Everyone who trys it is simultaneously frightened and impressed :)
Received wisdom is don't do it. It strips the joints and can lead to them falling apart. Infra red stripping is your friend though.
I've seen prices around the £40 mark but some people might just be throwing them in a bath of caustic soda and that can't cost the earth if you don't mind how its disposed of.