In most cases, the two can be used interchangably. For example, I do not see a difference in meaning between:
Sie gab das Signal zum Wenden.
Sie gab das Signal fürs Wenden.
Sie gab das Signal für das Wenden.
She gave the signal to turn.
Ich bin zu alt zum Fallschirmspringen.
Ich bin zu alt fürs Fallschirmspringen.
Ich bin zu alt für das Fallschirmspringen.
I am too old for skydiving.
However, I would consider zum here to be much more idiomatic than fürs, which is in turn more idiomatic than für das.
The first exception from this I can think of is when the nominalised verb has an additional meaning that the regular verb does not. For example, das Leben cannot only refer to the process of living but also to a life as a whole. So, there actually is a difference in the following example of yours:
Das Geld reicht zum Leben.
Das Geld reicht für das Leben.
The first variant can only refer to the process of living, i.e., the money we are talking about here suffices to sustain living, e.g.:
Mein Gehalt reicht zum Leben.
My salary allows me to live.
The second variant can additionally refer to life as a whole, e.g.:
Dieser Lottogewinn reicht fürs Leben.
This lottery win will last for life.
Something similar applies to das Essen, which may also refer to the whole ritual arond consuming food, while the verb essen is more narrow. This allows for a small nuance in your second example:
Die Menschen nehmen sich kaum noch genug Zeit zum Essen.
Die Menschen nehmen sich kaum noch genug Zeit für das Essen.
In the first case, people do hardly take their time for consuming food at all. The second case could also refer to people not taking their time anymore to sit around a table and dine but instead consume their food during other activities.
Also note that such substantiations can evolve their own meaning when referring to a special event. For instance, in a particular context, Fallschirmspringen in the above example can refer to a special skydiving event and not skydiving in general. For example, one could say:
Ich bin zu alt für das jährliche Weihnachtsfallschirmspringen. Aber im Sommer werde ich wieder fallschirmspringen.
I am to alt for the annual Christmas skydiving. But I will go skydiving again in summer.
The example given in Jan’s answer is another instance of this.
A second exception is when für means in exchange for or similar. Here, you cannot replace fürs or für das with zum. For example in the following sentence:
Sie gab ihm 10 € fürs Rasenmähen.
She gave him 10 € for mowing the lawn.
This is particular weird, if für can be mean both, in exchange for and to like in the following example:
Sie gibt ihm 10 € zum Einkaufen.
Sie gibt ihm 10 € fürs Einkaufen.
In the first case, she gives him money so he can then go buy something. The second case can also mean that he already went shopping and she gives him 10 € to compensate his expenses.
Finally, there are some fixed expressions, which require zum, e.g.:
Es ist zum Heulen.
It makes you cry. – It is pathetic.