Gold and money : when to spend it ?

Hello eBelgium community !

I don't know if this section is still in use, but I got a useful question to ask.

I am now level 15 and I have some BEF and gold. I got a mission (the iron mine and weapons factory thing) that wants me to spend much BEF's (iron mine) and gold (weapons factory).
The point is, when is it useful to spend our gold/money ? A mission that asks for it, is it a good enough reason to spend ?
If so, maybe you have some examples where spending gold/money isn't that useful and where we have to avoid those cases ?
And if not, when is it appropriate to spend it in order to complete the mission ?

Thanks for your future answers

The missions are good reasons to spend them not only you get more stuff also by doing this you can upgrade your town hall and that will upgrade your healthbar so you can wait longer before it's full and then you can do also more.

Missions are always a good time to spend money as the foreseen reward always compensates a good part of your investment, in this case being able to upgrade your town hall.

For gold: Spend it in a Weapon and a Food factory and upgrade those when special offers are available. Until you make enough money for yourself, there's no better way to spend gold.

BEF are best spent in Cattle farms or Saltpeter mines, again this is until you're comfortable with the income you have.