Mot failure 300s seem to sell for about £400/450.
Facebook selling sites are also worth a go.
But...
Chassis welding isnt that troublesome, compared to, say, sills & body mounts, or arches etc. Most chassis stuff is cut out the rot, and then let in a flat plate. There's not so many curves etc. involved, and access is generally good. People plate over rot, but, whilst that will get a pass, its temporary (i reckon 3 years). Do a good job and something else will write off the vehicle first.
But it will be the same for next year's mot also. If i had a fail [that i wanted shot of] id probably weld it up, get a pass, and then sell it with an honest description. A long mot disco 300 in a rough condition is probably still 800/850 - more if it look a bit shiny?