The desktop processors are preferred for the given reasons, but context should be taken into account. If you have to source and maintain a hundred computers you are likely going with Dell or HP workstations (which only come with Xeons) with an enterprise discount and service agreement which makes them cost-competitive.
RAM depends on what type of Civil3D work you are doing. 8 GB may be fine for basic work, but it must be very well organized and optimized. Not to mention you'll probably be running other applications alongside.
SSD is good, although you need as large a drive as you can afford. Many will skimp on size only to find they don't have room for the next release alongside their current one, plus this new program, oh and that looks cool I should try it and... well, you get the picture. Plan for the future. If at all possible get a large SSD for the programs and a second fast TB-sized drive to store stuff on.