For this I would pickup a refurbished Dell Optiplex or HP and a monitor.
Example...
computer for $109
Dell OptiPlex 5050-SFF Intel Core i5-7500 3.4 GHz, 16GB RAM, 256GB M.2-NVMe, Windows 10 Pro 64bit, DVDRW (Renewed) https://a.co/d/eeus4AW
monitor.. good cyber monday deals right now. depending on size will cost around $60 and up
microsoft office... if it absolutely must be microsoft office, try to find a deal on assorted discount sites. example newegg has MS Office Home for under $100. most people use quickbooks for bookkeeping...... however, there are FOSS applications that can fill this requirement to cut costs. LibreOffice is what I've been using for years, and GnuCash can accomodate most simple account tracking