I`d be taking this on as heavily oxidised pvc,the grey staining along the bottom of the frame in pic 3,the last one,isn`t going to shift with cold.If neccesary i`d even open a bottle of pvc solvent cleaner to shift some of the staining on the verticals if its not scrubbing off with hot and the strong alkaline cleaner left to dwell,(Master Cleaner from Concept Chemicals which is probably akin to G101 if not better).
If it had got that mucky in 3-6 months then it would be an easy enough clean but to me it looks like 6 years worth,possibly more.