Lot's of random advice from the internet. None particularly unique to Amsterdam.

So what do I need to pay attention to?

1. Product Execution. Putting something in front of potential customers and getting feedback and iterating repeatedly and rapidly.

2. Business Execution. Proven legal documents including vesting schedules.

How do I determine what a reasonable amount of equity is?

IMO, equity should be evenly split between cofounders. Google Joel Spolsky's essay. If someone puts up cash, treat it as a loan to the company not for equity.

What legalities do we need to get arranged?

Proven articles of incorporation.

Should I get my own lawyer?

Avoid people you don't trust. The only lawyering should be to create the corporate entity. If everyone is lawyering up and hard ball negotiating at the outset, no. Just walk away. It won't get better. Even equity avoids lawyering up.

How do I determine whether I'm good enough and whether he's good enough?

Track record. I'd look long and hard at the specifics of what an "idea person" really brings. Particularly experience with software products and a real world understanding of development processes. Cold calling developers off of Linkedin is not a good sign. Wanting a third person to perform back office operations for two people is not a good sign. Not being confident to learn and manage finance is not a good sign.

How do I select a software team?

Until there is proven product-market fit, you don't. Then slowly and carefully from your rollodex and those of the people in it.

How do I plan this? (My past couple of jobs were with big, slow moving organizations. A startup needs to move a lot faster than that, I assume.)

If it's really a startup, i.e. a company organized and focused on growth and providing returns to investors via increases to the value of equity rather than cash flows, it's not something that gets planned beyond grabbing the tiger tail and hanging on.

Can 40- and 50-somethings run a startup and still be in bed in time and take the kids to school?

Maybe. I suspect it depends on luck and the individuals and their situations. But if those things are core values, then don't give up on them.

How do I make sure I pick the right technology stack?

By focusing on it's immediate suitability for product execution as mentioned above. If the startup is successful, the technology will change do to scale and evolution of the product and everything will have to be rewritten and rearchitected.

I'm leaning towards Angular+Scala/Finatra, though I have no Finatra and little Scala experience yet. Is that wise?)

Absolutely not.

Are there any Dutch subsidies or VCs we should know about?

The idea person should already have this on their plate and be working on it.

What other things am I missing?

My gut is that there are elements of "getting a freelancer to build it for free" in the proposal. How much of that exists can be approximated by the degree of difficulty you would have building the product from right now without the "idea guy" and "back office guy" being involved.

I just don't see what the "idea guy" is going to be doing to create value while you're slinging code and the other person is handling the money and emptying out the trash cans.

Good luck.