Is there anything (for example in the T&Cs) that stops Google from creeping ads to paying customers in the future?

I bet not.

Yes. Google Apps business model, and all Google Enterprise products for that matter, do not rely on ads for revenue. Google sells their business customers a product and generates revenue through those sales. The TOS protects your data.

Perhaps you didn't understand my point.

Google could easily decide to serve ads to paying customers, and it seems likely that they've left this option open to exercise later.

The fact that they apparently "do not rely on ads" is irrelevant.

There are plenty of existent services that have revenue from customers, but still serve ads (visible examples include: pay-TV, public transport)

They could, but they would have to change their TOS, which business customers would have to agree to. Most of these customers would not, thus destroying this portion of Google's business, so they would never do that. Ads are not a part of Google's Enterprise business model. I don't know how to state this more clearly.

So what are my options if I can't use a service that can potentially serve ads? None?

Yeah, this is the problem. I pay for Hulu but there is no way to get out the ads. Google is going for an interrupt-based ad model here so you should expect it in the paid version too.

If they take example from AT&T, they would be selling your information even if you are a paying customer, which might result in more targeted ads elsewhere...