If Vdd is powered at 5V, can the device interface properly over SPI if the SPI bus is at 3.3V? What precautions should be taken?
The AD7943 offers superior speed and power consumption performance and is relatively a new part that will replace the industry standard AD7543. To answer your question, one is packaging. AD7543 and AD7943 have Plastic DIP (N-16) as a common package. If you have been using AD7543 and would want to change to AD7943, you can use Plastic DIP package option for AD7943. For AD7543, other package options are Leadless Ceramic Chip Carrier (LCCC), Plastic Leaded Chip Carrier (PLCC), Cerdip, and Small Outline IC (SOIC) packages while AD7943 have Small Outline Packages (SOP) and Shrink Small Outline Package (SSOP).
Another one would be the operating temperature range. AD7543 could operate at an Extended Temperature Range (-55C to 125C) while AD7943 would be able to operate at an Industrial Range (-40C to 85C).
If you would like to pursue the AD794x series, and want to operate at an extended temperature please take a look at AD7945. AD7945 has a 12-bit parallel interface and is available in 20-lead DIP, 20-lead SOP and 20-lead SSOP and has an operating temperature range from -55C to 125C (20pin Cerdip option).