You can add one change in running mode. Before stopping test, you can copy parameters of some component and paste this in this component after :)

Not think about money. Just install both engines and try to create prototypes by youself.

Unreal is better for FPS cause optimized for this genre. Unity optimized not so good (occlusion culling fixes little percent of problems, you need write optimizations by self, hide unseen rooms with scripts). Unity is engine just for arcades but not for seriously 3D projects (but graphics effects in Unity is similar Unreal). But with your little team, i think, is better to use Unity. It's easy to development, C#-logic and shaders compiled in split of seconds. Unreal need more time for this operations (shader created with blueprints can compiled in minutes on not top machines... and more details like this).

Unreal and Unity is different instruments, try they youself, but please, write here about result of your choise :)