Plasma Table Build Log Including Free Plans Link, & Pictures by Weldtutor here are some plans in this forum. Check them out. Also there are many other build logs here you should be able to find something you can use.
A plasma/router conversion can be done but you'll have to build the table for the router & use it as plasma. It's not as easy as you might think to convert. Main reason is the clean up process between the 2. Plasma creates loads of black gritty dust. On the other hand router work is almost a white glove project.
Personally I wouldn't even want a router table in the same building as a plasma machine.
After you read lots of the posts on the plasma section of the ZONE also check out the router build section. I found it very helpful in designing & building my table.