I don't think there are any 32-bit boards that meet those specifications yet. Someone will soon correct me if I am wrong [edit: see next post]. However, the Duet WiFi gives you 5 on board drivers with up to 256x microstepping, and the design of the expansion board for it is well under way. If you can't wait for the expansion board, you could hook up a Pololu driver on a piece of stripboard and connect it to the expansion connector on the Duet WiFi to provide the 6th driver - as some other users are doing already.
The Duet series is especially good for deltas because its firmware provides segmentation-free delta movement and fast delta auto calibration.
Edited 3 time(s). Last edit at 09/22/2016 08:03AM by dc42.
Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod
Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].