You can get the gaskets from:
Manifold to Engine - Gasket with fitting kit:
https://www.importcarparts.co.uk/parts-info?id=12009
Manifold to Engine - Just gasket:
https://www.importcarparts.co.uk/parts-info?id=12008
Manifold to overpipe / Overpipe to 2nd cat - Gasket:
https://www.importcarparts.co.uk/parts-info?id=12010
No, you don't need to.
You should only think about it if you track the car alot, but then I'd look at getting the manifold ceramic coated or have thermomet applied.