I am using WL500-g and WL138-g in network with computers running W98 and W98SE W2K and WXP. I think installation and setup is very easy.
Here's the HOWTO:
Put software / driver / documentation cd in computer
Read install section of printed manual supplied with equipment or from file on cd
Install software and equipment accordingly.
Read relevant parts of the manual and configure to your needs.
If everything works as you want, leave it as it is.
If you want to play around or need specific features this forum is a very good place to start.
Find another dealer
You may wish to check on the ASUS download pages if they have drivers / firmware with W98 support for the equipment you want to buy. For the models you list this would be:
http://www.asus.com/support/download...0g&Type=Latest
http://www.asus.com/support/download...8g&Type=Latest
http://www.asus.com/support/download...7g&Type=Latest