Honestly, I see no reason why you cannot go to your finance person and tell them an 'IT professional' has offered to help you pick out components and to further assist in the actual building of the computer (via online assistance here)...
Tell them that this person is very knowledgeable, and that they are helping to make your money go much further than it would via any other route.
Further more, they (this IT Professional) have strongly suggested this route as it will save you lots of money many years down the line (from incremental upgrades over having to buy a whole new computer).
You can go even further and say that this person has kindly offered to help you as an act of compassion (as they are aware of your disability, and your financial situation), and that normally this service would cost significant money (the selection of components and building of a computer).
And as has been suggested earlier, you can *correctly* state that as they are assisting you and not doing it entirely for you, that you yourself will be learning a great deal in the process.
The next step for you to do, and you should do anyway... Is to determine for yourself the components you need within your budget.
So if I were in your shoes, I would hit YouTube and *research*... It may seem tedious, but you will both learn and be kept occupied... And trust me, the sense of satisfaction you will feel after building your own computer will far outweigh any joy you could feel from just taking home some inferior machine from the store shelves.
Just remember to check back here once you think you have a build. Don't impulse buy. Check, check, and check again. It's really easy to build a computer if you've chosen the right components... It's the choosing of components that can be tricky... But I have no doubt you will be able to do it (with a little assistance from us).
I was completely green when I built my computer years ago, and I didn't have any assistance. I was completely boggled when I began, but all that was really required was a lot of reading.
The only mistake I made was to buy a 'factory overclocked' graphics card.... Which ended up burning out a few years later. It seemed like a good deal, but based on experience, I wouldn't trust 'factory overclocked'....
... And on that note.... Each and every component you choose, you should read a number of reviews on. I failed to do that on the graphics card.
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