1) We only support Linux. 2) We only support Vulkan (or OpenGL, but I prefer Vulkan) 3) Singleplayer only. 4) Engine features: RGBA Diffuse, specular, shadow, normal/bump, emission maps for materials only. A simple triangle rasterizer with forward rendering. Support for audio, dae animations/rigs/geometry. Physics collisions through an octree and capsule and basic linear algebra. Basic first person/third person cameras. 4) As much content as possible from copyleft/free sources. Mixamo, Cgtrader, poliigon, textures.com etc. A programmer's job is to use whats out there. But, given an extra 6 months, I could also model and texture everything myself as well. I'll do all level design. 5) Dialogue written by me, voice acting through AI or vocaloid. 6) I would estimate the final code to be around 50,000 lines for everything. I can give you a breakdown if you want. All code is in C99, minspec is a ryzen 5600g, which is like $150 max these days.
Something I've missed? I've described a classic first person adventure shooter like Halo for you there. Simply let me know if I should do something else.