Welcome to the forum!
It will all depend on what make of Countach replica you have and what the original builider did when they were putting it together.
I had a MasterCo Countach and there were loads of little jobs to do on both the interior and mechanically in order to give me the car i wanted.
Best thing to do is get it home, post loads of pics for us to see
and then start deciding what bits you want to tweak.
Ebay is a great source for some parts and there are lots of alternatives to OEM that can be bought dirt cheap if they're not already fitted. i.e. the ashtray is often left out, but an old Jag XJS one is almost exactly the same as the Lambo one and is hugely cheaper!
Or if you want original then have a look over on https://www.eurospares.co.uk/
and i wish you and your credit card the best of luck!