Since this is only a non structural piece you can "glue" with a panel adhesive from a local parts store (SEM, Fusor, 3M, etc). Cut the rust out, get a panel flange (pneumatic) tool from harborfreight.com for $40-$50 and glue the panel in. Perform the needed body work and paint. Should have less than $250 invested. the SEM website has good detailed instructions on how to do this easily.
http://semproducts.com
http://semproducts.com/images/Users/...20ADHESIVE.pdf
Personally I have a welder to perform the job, but the panel adhesives are used everyday in many body shops even if they have a welder.
Paul