This my two dollar contribution. It used to be 2 cents, but the inflation...
Is it possible for checking online which drivers need an update before or after updating to windows 10 coming from windows 8.1.
Yes, write down all hardware you have (including chipsets). Visit the manufacturers websites of the hardware and see if they have drivers for Windows 10.
Or does w10 automaticly update drivers needed with the update process?
As with previous versions, Windows 10 comes with a lot of drivers and will install them if needed. It can always be that Windows 10 doesn't have a driver for certain hardware. In that case you will need to look at the manufacturers website. Keep in mind that there are millions of different hardware. Look at e.g. at the amount of different printers there are. Putting all drivers into a installer is not possible.
Does MS guarantee a working pc after updating? (usb ports, card reader etc)
No. As I said before, the installer doesn't have drivers for all hardware that exist. Besides that, there can always be something that went wrong during the installation. Think about a power dip (in the system). It can be so short that you don't notice it, but it can be a 0 instead of a 1 is written to the hard-drive, a setting file or....
How to fix any issues after updating.
Same as you need to do to fix issues before updating. Find the cause of the problem, find a solution, apply the solution.
Has time come to update now, or is it better to wait?
Personally I would wait. Windows 10 has just been released and it would not surprise me if soon bugs, security issues and things like that will be found in it. Look at all the updates/patches/service packs that where released for other Windows versions.
Besides that, a lot of software is not or not fully compatible with Windows 10 (yet?).
I sure would check the developer websites to see if the software you are using will run on Windows 10 or if there is a version available that will work on Windows 10 before upgrading the OS.
And as it is with all OS releases, it is a good practice to cleanly install the latest version and not perform a upgrade.
Upgrades will often (almost always) leave things from the ol(der) OS behind.
