I suggest that if you're going to use a plate, for example to install a lift, or a heavier duty router later, a double layer table is a good idea. You could, for example, use some Baltic Birch ply gluded carefully down to a 3/4 layer of MDF. cut the hole in the ply slightly larger than the plate, then cut out the MDF about half an inch smaller. Then you can use some commercial "levelers" to bring the plate exactly level with the top. The double layer adds to rigidity and flatness, which is far more important than whether you elect to use, or not use a plate.
If you elect not to use a plate, you can remove the bottom plate on the router (the base) to mark the location of the holes you will drill to mount your router. If you can't find a pre drilled plate, you can use this method to drill holes in the plywood.
Another reason for the plate, is that many standard-drilled routers have plates pre drilled for the mounting bolt pattern. And, most plates have inserts that allow you to minimize the space between the insert and the bit to minimize tear out, and so sawdust can come out.
I also recommend you get an aluminum plate that has a twist lock or bayonet type insert so you don't have to unscrew (and misplace) three tiny bolts every time you want to adjust the router.
The matter of a lift: Once you have one, you will rapidly tire of fiddling with your router to get the bit to the right height. It is much better when you can adjust height from above the table, without having to remove it every time.
I will also add that the best setup I've had is with the Triton 3.5 hp router in my table. Yes, I also have a plate, in fact, the expensive Woodpecker plate in the pix below, which is 1/8th thicker than most common plates. No sagging. The Triton has a built in lift (you remove a spring) and significant safety interlocks, and the router cost is about $240 or less, which is less than the cost of an typical special purpose router lift. It is heavy, so you'd want to keep you other router for free hand use. See the picture of the Triton in a plate with the top height adjustment crank in place.
All this may be more than you thought you wanted to know, but often, buying cheap backfires. Not always, but often enough to motivate me to give you a heads up. I also added a picture of the levelers as installed in the table. They are Kreg brand, so they're really nice. They will make building your table work better. Don't forget the two layers.
The more I do, the less I accomplish.
Last edited by DesertRatTom; 09-07-2018 at 08:25 PM.