Yes if you want to vary the speed you need a VFD but the sames goes for a router as well
Spindles come in different diameters so you need a mount like the one in your pic to suit.
The mount bolts to a z axis which will bolt to your sled
All of the above can be purchased cheap on ebay
Something like this
A good example is here
Im building a sled with a 1.5kw spindle but I dont need that much power for a maslow. Its just that I had one spare. If I was geeting another I would have got one in the 0.5 to 0.8kw range.
While you cant directly compare the power rating of a router and a spindle a very rough guide is spindle power
x 2 or 3 = router power.