>>14164 > need mainframe for filling out 1040 form Nah. Most people in the 80's just used the paper forms and a calculator. When I turned 18, my aunt showed me how to fill out the form and I've done it that way ever since, even when I did contract work (form 1099) and had a bunch of deductions and investments.
But if you did want the computer to do all the work, it wouldn't need to be any kind of fancy computer, just something powerful enough to run a spreadsheet, because that's the kind of basic math that taxes are. Actually here's an "official" software for DOS:
https://winworldpc.com/product/turbo-tax/9x Needs an 8088 CPU, but I'm sure a Z80 could handle it just fine too (I mean it's capable of doing those calculations, not that it can run DOS software made for Intel x86). Anyway a plain old desktop calculator can do taxes, so you really don't need much.