What is needed.
WHAT AN ENGINE PROVIDES
Rendering Physics Audio Input Asset management Scene editing Build and deployment
WHAT OTHER TOOLS ARE NEEDED
Image editing Model creation, for three-dimensional games Audio editing Version control Task tracking
WHAT VERSION CONTROL MUST HANDLE
Large binary assets, which ordinary source control handles badly.
WHAT SOLUTIONS EXIST
Large file extensions for standard systems Systems designed for binary assets Locking, so two people do not edit one asset
WHY LOCKING MATTERS
Binary assets cannot be merged, so simultaneous edits lose work.
WHAT FREE TOOLS EXIST
Capable options in every category, sufficient for commercial work.
WHAT TO SPEND MONEY ON FIRST
Nothing, until a limitation is actually reached.
WHAT ASSET STORES PROVIDE
Models, sounds, effects and code, purchasable.
WHAT TO CHECK BEFORE BUYING
The licence, and whether it permits your use Whether it is maintained Whether it fits your art direction
WHY THAT LAST POINT
Mismatched purchased assets look worse than consistent simple ones.