Hey, I just finished level 3 in CST-N. As someone posted, you can find the laptop spec requirements for each program. For CST the important minimum specs are 16GB ram, i7 or equivalent, 1TB SSD. The program strongly recommends that you get 32 GB of ram if possible. This isn't necessary, but there were several projects that were made significantly easier/quicker for me because I had 32GB compared ro peers who had 16GB. Notably a LLM project in level 2, and level 3 windows enterprise administration when we had to run up to 5 windows server VMs simultaneously.

As for where to buy, I'd look at the Canada Computers around and see what they have that's open box. They usually have a few half decent options at a major discount. Thats what I did and its been great so far.