What a ripper The car that is.
We are spoilt here in Aus with minimal rust and I can't see yours as a problem.Thats nothing to repair to keep a mk1 2door from the scrap heap.
Anyone who gets one without floor or front pillar rust is very lucky.
I wouldn't go silly with seam welding as it is very time consuming although some areas up the front would help particularly the turrets.
If you are putting in a cage weld it via brackets to the body.
Bond Roll cages still have the jigs for mk1 Golfs and may save your man a lot of bending.Buy a Bond in pieces and weld it in .
I ve done it this way and saved heaps.
Got to go will help whenever I can.
Ed

ps I've got one here I'm restoring and it is worse for rust but very straight otherwise.
They don't make them anymore