"DO NOT BUY A OLD USED COMPUTER, there can be trojans in the firmware, or exist a SMM backdoor. also do not use any closed source(Windows, Chrome, ...) use linux or bsd. do not use proprietary/binary driver for your GPU, there can exist a backdoor there too.
Once again - if you *never* connect it online it doesn't matter how nastily infected it is (and physically disabling any ability for it to ever connect is my advice and what I have done to the old laptop I bought for this purpose).
Also don't use the OS on it either - use a LiveOS (and one that doesn't use the HDD).