Related to this, I recently started with a fresh GTA install and downloaded RDE, which has TONS of different cars but all in vanilla, lore-friendly form. Honestly, if the lightbars weren't such garbage on vanilla vehicles / if vanilla vehicles were ELS compatible, I would have just kept them. I personally don't mind having real-life cars in my game, but most of the time I can't stand having real-life department liveries in my game (the exception being if there were liveries for my university, or a local PD or something)
Overall, great idea but impractical. I have no experience in ZModeler or anything like that, but it just seems like it would be a ton of work to install all the extras to default vehicles. My two cents.