I disagree somewhat. This will play ETS2 and ATS just fine and is within the OPs budget...
https://www.pcspecialist.co.uk/saved-configurations/amd-am4-home-office/Fy3NncUn75/
6mb/s on e.g. Steam is 6 Megabytes per second. Task manager is in bits, So, 6x8 = 48. Near enough to 50 mbps. You are downloading at more or less max speed. Use ethernet or get a new modem if you can get faster speeds from your ISP.
If you log into your account and look at your orders then you can amend/add to the order as long as it's in pre-production. Any additional cost will need to be paid for by card. You can't add to the finance.
You could buy it with minimum memory and add the 8 x 32GB yourself. Wouldn't cost much more. You need to change Windows 10 to the PRO version as home doesn't support 256GB.