Wow, been under lots of B segment cars, never seen one with a subframe rusted out like that. That looks like what Hyundai/Kia stuff does.
If the rest of the car is decent, you can replace the subframe. Not a huge job. Shoot, I just put one of those in a scrapper last year, too (from a crash car I was parting out). Never saw the need to keep it.
I'd not lose a lot of sleep over it, as it probably has a ways to go before it becomes an issue that would cause a problem so long as the mounts for the engine/trans and lower control arms are still secure.
These are not TDI specific, any 4 cyl B5 will work (according to ETKA).