There have been reported issues of fitment issues with some Invidia headers, but not all. Each pan comes with an OEM sized drain port as well as one 1/2 NPT Port. You will need to get a plug for the NPT port if you don't plan on using a quick drain valve or anything else.
OEM Belly pan will need to have a small piece trimmed on some models. Turbo EJ20 engines will need to change to the STi dipstick and tube or simply mark the old dipstick with a new full mark. The item "Killer B Motorsport Performance Oil Pan Sump FITS Impreza WRX / STi EJ20 EJ25 UK" is in sale since Wednesday, August 21, 2019. This item is in the category "Vehicle Parts & Accessories\Car Parts\Other Car Parts".
The seller is "funky_rims" and is located in Dundonald, South Lanarkshire. This item can be shipped worldwide.