Keep or Replace PII computer

We have a few Compaq Deskpro Pentium II 266mhz computers that are coming to the end of their 3 year lease. I am looking for suggestions on why I should replace these computers or purchase them under the 10% buy-out option? The software used on thesecomputers is MS Office 2000, ACAD 2000, and building design software.

Thanks.
S Karmann