It seems that you will not be the first to have this issue, that is one reason why I prefer to drop the oil out via the sump plug - and while performing an oil change that way, you should be checking over the underside of the car, and so catch this issue early and hopefully stop or slow it down before the sump rots through.
Many years ago, Ford changed the paint on their smaller engines from durable black paint to grey paint, my wife's 1992 Ford Fiesta's sump caught me out, and I replaced it before it became porous - but, I wiped the new Ford sump with meths to degrease it before re-painting it with black Hammerite, wiping it with meths removed all the thin layer of grey paint!
So, maybe you should degrease your new one and smother it was a small tin of Hammerite paint.
Actual details covering removal and fitting I can't give you, though make sure that you know if a gasket or liquid gasket is used to seal it, I'd think that it would be liquid gasket nowadays.
Edit:- it looks like your steel sump part is just the "lower sump" ie really just a closure to the bottom of your engine, if that is correct then it should be slightly cheaper, p/n for CPTA version of that engine which claims to be the same as CHPA, is 04E 103 602 B and the sealant is listed as being D 176 501 A1. Now, at the age of your car, maybe it makes more sense to source an aftermarket equivalent for both these parts, I've never needed to look for aftermarket steel sumps so I can't
help there - but a good local motor factor should be able to find one for you, though maybe phone your local VW Group
dealership parts department for a price and to confirm these part numbers.
