Tabbs,a revolving centre allows higher speeds i.e. if you're cutting small diameters or certain materials and requires no lubrication to run.A steady is for long parts and to support components a moving one from what i can remember allows the diameter to be turned with the steady following behind.4 jaw chuck allows you to clock things up true,if you have a concentricity tolerance or want a bore to run accurate to the diameter,far superior to a three jaw chuck.Wouldn't bother with a digital speed display might be worth getting a D.R.O.(digital read out).It's so much easier for turning as you can touch on,measure diameter and put this figure into the dro.Material dependant it should come off in curls(aluminium for example) but can usually tell by sound which will come with experience.Speed wise there's a formula you can use which uses cutting speeds of the material which should be available online.I'll have a look around now for you