If the battery has not been removed already check it out, barrel batteries leak over time slowly destroying the motherboard and surrounding components.
You can still buy network cards for A4000. Tad expensive at £75 though, but a fraction of the price they were in the old days.
Easiest way to install AmigaOS is to use WinUAE and put the hard drive (or compact flash card) in the PC. That way you can install OS3.1, upgrade to OS3.9, install all Boing Bag patches, install any tools & games (like WHDload) in a very short space of time (with backups of your disks in ADF format found on the internet

).
I would recommend that you get an IDE->CF back panel adapter anyway to make it easier to transfer files from the PC to the Amiga without a network card. They are dirt cheap, about £3 for the adapter and another £5 for a 2GB CF card.
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