As you wait for your system to arrive from Ukraine, MotoRacer, the most valuable thing you can get your hands on in the meantime is a copy of Norton Ghost 2003. (Ghost 2002 should also be okay, but I don't have experience with it.) And an IDE DVD-RW drive and spindle of DVD-R.
The first thing you should do is install Ghost and then create a backup image of your system to a DVD-R (it should fit on one DVD-R unless the seller has loaded a bunch of other software). Now, you'll always have a "fresh" copy of your system to reinstall if stuff goes wrong.
And stuff will go wrong: borked installs of software, deleted drivers, etc. 98SE is a great OS but the most innocuous things will mess it up.
I love using Norton Ghost with my retro builds. It takes about 20 minutes to create a 2GB image and about the same to restore.
Doing this would be especially important if the seller did not provide you with an original copy of 98SE and the motherboard drivers.
With that being said. Windows 98 can go online as far as I know, but if you do take it online I wouldn't recomend doing anything finacial based. Only risk information you're willing to loose on it. It's will be easier to pick up a load of virisus on that compared to Windows 7,8, and 10.