Read this site first and or sites like it.
Your concern was about being ripped off and how to guard against it, and you are not alone. It's possible to do what you have described but it is a lot of work for little gain and generally you will have no problems. If you're still unsure, take it to a computer repair store and they can test the parts for a fee. These tests, with general questions - "where/when did you buy? Any major/minor troubles, change any parts?" will help you.
Run this free test, which tests the memory and gives you details on what is installed: memtest86
Place it on a CD and run the test. Most info comes up very quickly and it's a fair test to run anyway.
Start computer and press delete or F1 to open BIOS and check the info and that it matches.
On the users desktop, click on the menu, then accessories, then in 'Run', type 'cmd' in and tap enter or type 'cmd' in search. Click on it to open.
- In the cmd panel type - systeminfo - this gives you the system info.
- Also in the cmd panel type 'dxdiag' and click "yes" to drivers. It will need an Internet connection (if you want to know if the drivers are signed). This also provides more info, such as sound, system, and display.
From the Internet, you can have a test run and go to the 'one care scan' Microsoft site. Click on the full scan and they will ask to test if it is a legal copy to do the scan. Click 'ok' and you will not have to use the scan. It tests before the full scan starts.
You can open the machine and look at the stickers to make sure they match system info (small screw driver set required). They will be in good condition as they are not exposed to sunlight. Is the age about right? There is only so much you can do.