The oil leak is most likely the rocker cover gasket being dried out and leaking, or being incorrectly fitted.
If you have no fogging of the screen so far, then it could probably be reused. A new seal is a must.
The replacement panel is about £45... I just bought one. Thats just the outer skin though. Depending on if the corrosion has crept deeper. Have a look in the boot underneath the rear shelf, any deeper corrosion is easy to spot there.
I don't know what a bodyshop would charge just to replace that outer skin, remove and replace the screen and paint, probably a good £500-600 easy.

Cheers,
Ben.