Core drills generally come in two forms: - 1) The conventional chuck type takes a hex arbour. 2) The threaded tool holder type takes a screw fit arbour. The former, allow you the flexibility of a standard drill. The later, higher torque of a direct drive at the expense of more laboured tool changing.
Of the chuck type the Makita 8406 is fine example, offerings from Dewalt and Bosch are better, but my personal favourite is the Milwaukee DD2-160XE.
Direct drive types are the professional choice however. The Marcrist DDM2 and DDM3 allow wet and dry cutting at fixed speeds. I have never felt the need for variable speeds when using one of these, I find them remarkably efficient when paired with the same make of core cutter, but a few of my colleagues prefer the Milwaukee chuck type. Most of the main players have top of the range models, with the Hilti being the firm favourite with the majority of my work mates.
What ever you decide to buy, just make sure you are not tempted to use an SDS drill to drive a large core cutter.
The SDS tool holder is very inefficient when used in the hammer stop mode, a large core drill will quickly stress and wear the tool holder and ultimately destroy a 4kg SDS if it does not have thermal protection. If you suspect your drill of getting excessively hot, stop drilling, switch to the max speed range and run it flat out for least 5 minutes in free air to allow the motor fan to cool the windings.
