First of all, you must know that helios is a generic title for a laptop line that has anything from i5 8xxx to i9 12xxx and from 1050m to 3080m with many combos of the cpu/ram/graphics card, and so we cant really advice one way or the other.
Second, since you seem to know, I assume you realize that everything is about risk/reward. Getting better specs with a used pc/laptop at low prices (reward) comes with the uncertainty that maybe in a few weeks it will go bad and then you will have to pay more for parts (risk). This is very subjective and no one can really answer. $350 for someone who has $3.500 budget is only 10% and so an acceptable risk, but for someone who has a budget of $350, it's 100% of the budget and so... not a good risk, just to state the 2 extreme examples.
And even if you can repair it yourself as you say, you have 3 issues,
1. Finding parts: not everything is available as often you need VERY specific parts that are made for a range of models only.
2. Some parts can not be replaced (again depends on the laptop) and need a total motherboard replacement.
3. Price of said parts.
My advice, since you seem to like the idea, stress test the laptop (game or benchmark) for at least a couple of hours, check its thermals and battery for longevity, and check available replacement parts in the market to have an idea of cost (rams, disks, CPU, GPU, mobo, monitor). If it passes the above... consider the risk/reward. If the owner says no to testing, you have your answer.
Best of luck.
