I have run some forums in the past, I don't think deleting user accounts and posts is really going to make a difference. It's the attachments which take up the space.
I also think it's going to get very tedious very quickly to do, and you'll quickly tire of it.
If you tell me what your hosting requirements are, I cold probably host it for you for free.
Finally, as long as the cars exist and people enjoy them, there is value to this forum. I'd hate to see it go away. Facebook groups are ok for 'in the moment', but cannot replace a forum's historic collection of knowledge.
I never got contributing here because I never found the right car (still don't have one, still looking). But I still enjoy the forum.