From reading your posts (and also that little segment in Golf+ magazine you did a while back) its clear you adore this car, you'll regret selling it so DON'T!
Replacing a sill isnt the end of the world, pm me if you want a quote for the repair and if you dont mind driving to b'ham for the work. We also have a mars red (pink) 1600 mk1 gti with complete and good condition engine/box ready to strip and scrap if there are any other bits needed.
If you can afford to run a mk4 as a daily and keep the mk1 for weekends then do that, or get something cheaper for a daily (mk2/mk3).