PC Building Simulator

A quick way to earn this if you don't yet have enough cash to spare on upgrading the office PC is to wait until you get a customer job to build a PC over 5,000 3DMark score.

This site is useful for calculating 3DMark scores when building or upgrading - https://pcbuildingsim.magn0053.dk/calculators.html
(Thanks to Cecil Kilmer for posting this link in another solution)

Once you have built the PC, pick up your office PC and put it on your workbench, then drop in the customer PC in place of your office computer. You'll then earn the achievement for having an office PC over the 5K score.

Then, pick the PC back up and place it in the delivery area for the customer to collect and replace your original office PC.

When you have gathered enough money in career mode you can start spending it on upgrading your office PC. You need to install 3DMark Advanced Edition and run it to get a score.

I managed a score of 5723 with this setup:

Motherboard: GIGABYTE GA-AX370-Gaming K7
CPU: AMD Ryzen 7 Eight Core 1800X
GPU: MSI GeForce GTX 980 Ti GAMING 6G GOLDEN
RAM: 4 x G.SKILL Ripjaws V (Black) 16 GB 3200 Mhz
Storage: 2 x ADATA Ultimate SU650 960GB
Storage: Seagate BarraCuda 4TB 256MB Cache
Power supply: Shean Powerking 600
Case: Cooler Master COSMOS C700P

I was able to purchase these parts on level 7.