Brain dump: In Unity, should I buy an asset or make it myself?

I follow these rules:

1. If it’s a core competency, make it yourself.
2. If it’s obvious to the players, make it yourself.
3. Your brain takes priority over these rules.

For example, let’s say I’m making an FPS game with portals. How much should I buy and how much should I make?

• Character movement is unique to each game and is essential for your FPS game to feel just right. So I consider it a core competency. I make it.
• Sticking the character to the ground is really more of a basic engine feature; more of a better CharacterController than anything. So I buy it (Character Movement Fundamentals).
• The FPS system in general. If we can buy movement assets then what’s stopping us from just using UFPS? Well, it’s an FPS game and so it’s a core competency, so I make it.
• The portal system? There are many assets on the store that does this already. But no, this is a core part of my game and what makes it unique. Plus, making it myself allows me to add functionality and make it FPS-ready more easily. So I make it.
• Markers on enemies? Aside from them not really working through portals, this isn’t really a super important core part of the game. If the portals weren’t a problem here I would buy it.
• Finally, for graphical and sound assets, it really just depends on how much players will notice that it’s bought. Maybe I can modify them too.