It will funktion the way I imagine?

what will happen if I need to reinstall OS, will stay the apps saved and ready to go?

Used normally and properly, yes they will. If you want to associate file types with any of the apps or pin them to the taskbar there are a few little things to be careful of, but otherwise your apps will stay on the second drive with all their data, and could be run from any computer the drive is connected up to.

Even if apps are installed on intern SSD do they need to be manually closed before Sleep/Restart/Shutdown?

Yes they need to be properly closed. But you can set the PortableApps Platform to close all apps on shutdown/restart which will take care of most of those situations.

On sleep probably not, but it depends how the second drive behaves - if it completely disconnects on sleep then some apps may have issues when the computer wakes up again, and the PortableApps Platform doesn't handle these situations as far as I am aware.

The most I'm thinking of, do portable Apps have the same performance as installed apps?

It depends on the app. Most you will see no difference, some you may notice a small delay when opening the app.

Very rare apps you may see a bigger delay, especially if you change some settings for the way they handle caches and other locally stored data.