Hi
Firstly there's an ADC to DVI adapter, the cheaper of the two, which allows you to connect a DVI monitor to an ADC port. It's basically just a cable that extracts the DVI video signal from the ADC port.
Secondly there's a DVI to ADC adapter, the more expensive of the two, which allows you to connect an ADC monitor to a DVI port. It includes a power supply in the form of a white 'brick'. It's available from the online Apple store (UK) for £69 (search for DVI to ADC).
All the G4 towers, with the exception of the FireWire 800 models, use an original Apple Airport card (the FireWire 800 models use an Apple Airport Extreme card instead). Unfortunately the original Airport cards were discontinued some time ago and sell for a small fortune on eBay. I bought one fairly recently and paid much more than a brand new Airport Extreme card would have cost from Apple. Alternatively you can buy (Mac compatible) PCI cards such as the Sonnet Aria:
http://www.sonnettech.com/product/aria_pci.html
Assuming you're UK based, the following sites sell Mac compatible hardware including the above:
http://www.macupgrades.co.uk/store/
http://www.macheaven.co.uk/
To watch TV you'd need some form of TV tuner, either in the form of an internal PCI card or an external USB device. MacHeaven seem to sell some, although the earlier G4 models don't have USB 2.0, so an internal PCI card or external Firewire device would probably be a better choice.