There are 3 things with this being completely honest since I use the software myself not in a Civil Engineering aspect though. Since your building it, is this his own business, contractor or part of a company? Truly no matter what I say below... pre built desktop or laptop is the best thing in a business environment unless your the one keeping on top of the hardware/software. If you can do not worry about the HDD/SDD or Ram in the rig when building.
1) Get a pre built system it basically runs out of the box, minimal need to do anything except just some quick updates. As well getting proper business class warranty/service for quick turn around if something happens.
2) Prebuilt business grade laptop. These are just as fast or nearly as fast as the desktop counterparts. Best part its mobile! Might be good for trips into the field if its required. Unless hes 3D Rendering for hours on end, I'd lean this way myself.
3) Build your own, you will most likely have to service it if anything happens but can save some money or increase the performance for a similar budget.
Now note on the HDD/SSD/NVMe/M.2 drives and ram.... It is typically WAAAY cheaper to buy it yourself and replace what comes stock! Unless there is a deal going on buy it on the side and do the upgrade after getting it. The rest of the system your stuck with what ever you get so don't skimp on it. Ram wise pending exactly what he does 16-32GB of Ram for the PC should work. I know point clouds (3D Scans) uses a lot more memory and if he uses that 32GB highly recommended, maybe even 64GB! GPU is that odd ball and without really knowing what he exactly does and if the software can take advantage of it, you might be able to get a lower end professional card, or might need the best you can get.
For me I use AutoCAD, Solidworks, Inventor, Recap 360 Pro (3D point scans), and Creo on a roughly $2200 laptop for work. It does a fantastic job of meeting my daily needs but its totally possible they could of gone with a gaming laptop and saved $600 and work just as well if not better too. Though if it was me personally I can run stuff easily on gaming grade equipment too, especially the graphics cards, get better performance and still be cheaper. I actually run a lot of similar software except Creo on my home PC and it crushes the performance 3D wise even with the non proper graphics card. Though that being said certain programs/functions require the special professional cards (NVidia cards mainly) to help speed up caluclations and/or renderings. AMD FirePro line is still not widely spread in the professional front but typically a lot cheaper, just double check to make sure its supported with the software he is using.
What type of PC does he have or has used in the past? That would give a good indication at least where to lead you.
