The cheapest and most reliable would be to run a wired network connection all the way up 2 floors, and especially as its running an old W98se operating system.
If you have the slightest possibility of having a 3rd computer at any time in the future, it's worth getting a router ~ peer-to-peer between 2 PCs can become a bit fraught.
Such are the economics of mass production, that you will find it cheaper to buy a bog standard wireless router which allows you to turn off the wireless aspect, for around £50.
A PCI Ethernet card is less then £10 and ethernet cables ought to be less then £ 5.
Printer sharing is cheapest done be connection to a PC which has to be switched on for the other PC to be able to print.
Getting your existing printer to connect directly to the network would require;
a) a more expensive router or
b) a separate printer-server "box", again more cost, or
c) a new printer which has a built-in network card.
On top of that, you will need to configure your network for the printer setup.
Hope that has helped a little?