DIY washbasins, what accessories do we need?

I know it's a dumb topic, but we are really not born craftsmen. When it came to the bathroom fixtures, the general contractor didn't have what we had visually imagined. Therefore, we agreed to do the washbasins for the bathroom and toilet ourselves. Now we have found some on the internet that we like. The washbasins are fully assembled, which is already convenient for us. They also include sinks. The sinks have holes (according to European standard size, it says), and that's it.

And with that, we're overwhelmed again. What else do we need to buy? The general contractor installs the connections. Quote from the contract: the pipes such as drain pipes, hot and cold water, as well as 2 corner valves, remain part of the contract.

You surely need that drain stopper in the sink where the water flows out. Also, we probably need the pipes from the hole in the sink to the wall. And we need a faucet.

Do we need anything else? Sealing stuff? Oh yes – we want to have it installed, not install it ourselves – unless installation is very simple. Does a faucet come with the necessary connections to the corner valves?

You actually don't need anything else there... Washbasin with fittings and siphon.

You actually don't need anything else... Washbasin with fittings and siphon.

How is it with the drain valve? There are some fittings that say "with drain valve" - is that then the part that goes into the hole in the washbasin? How can that work, the fitting doesn't know which washbasin I buy? Or are these holes standardized to such an extent that it always fits?

Yes, the usual drain hole has a diameter of 45mm.

Yes, the usual drain hole has a diameter of 45mm.

Quick question: This refers to the hole in the washbasin countertop, right? We are planning to buy the countertop with pre-drilled holes soon, and then I would have the tap hole drilled at 35mm and the drain hole at 45mm, correct?

You basically don't need anything else...
Washbasin with fittings and siphon.

But that already consists of dozens of parts that you assemble. Ok, you can certainly buy that as a combo, but then there would still be the gasket and this nut... or not?
When I once bought a washbasin, there was nothing included. Niente. A washbasin with a hole for the faucet and two holes at the back for the dowels. Those also had to be bought separately...