If you buy W2K at retail it is expensive. If you buy a used machine with no operating system installed, as I suggested earlier, most used machine dealers (ask an enthusiast to suggest a good store near you - can't help unless you are in Southern Wisconsin!) have a license to sell, as part of the sale of a machine with no operating system, a complete cd set from Microsoft of W2K for about $80. You might need help loading drivers, but probably not. Most desktops from major brands have good drivers as part of W2K. Many laptops need special drivers. This is not hard, but there is a learning curve.
Buy a machine coming off of lease about two or three years old. The corporate guys lease a lot of PCs and there are tons of them available, I mean hundreds of pallet loads. Most go overseas because Americans always insist on brand new. Buy them for $110 to $220. A thirty day warrantee is the usual. Get the machine running right away and make sure all functions work and then run it constantly during the warrantee period. These machines likely have many years left with little or no maintenance. Most failures happen early in the machine's life cycle, then comes a usually long period of no problems, then after that failures pick up again.
There are many ten year old machines out there that work perfectly, but require special hard to find drivers if you get earlier than what is called Phoenix Bios 4 version 6. This is a gadget inside the machine that installs the initial program so that the machine can do stuff like read the hard drive, keyboard, and such like.
With Windows XP, it "phones home" to Microsoft, and will prevent you from running one copy on two machines.
If you want these systems to run for five years, then you pretty much have to get Windows XP. That's the only one that will likely be supported with security patches for that long. The others such as Windows ME and Windows 2000 will probably be end-of-lifed inside of five years from now.
Running a Windows box on a DSL line without up-to-date security patches would be like living in Harlem 20 years ago without a lock on your door.
And if you haven't already, be sure to use a firewall. Something like the Netgear Model RP614 4-Port Cable/DSL Router, available at Newegg.com for $40 after rebate. See this reviewed at CNET Review: NETGEAR RP614 cable/DSL Web Safe router gateway .