She doesn't want to change to win10/11
I'm very reluctant touching the software installed as it's all expensive stuff with licensing that goes 10+ years back that cost thousands back then, pretty sure nowdays all this stuff requres monthly/yearly subscriptions.
I am questioning her business sence first of all, and second, this is not an operation i would personally do. I would explain and state the current situation several times and ask her to hire a business to do it for her, you know with warranties and such things. Even new hardware fails sometimes, and i would not even remotely want to stand in the crosshairs if something happened.
I fear you will have lots of headaches with this down the road my friend. The energy this gives me is just bad ju-ju. If you really have to do this though:
1. Research and build the newest machine within her budget. Buy quality components for stability, the best Power supply unit you can find even if it is way overkill. Add overkill RAM, we do not want the SSD's to get tired fast. It is the cost of doing business really, but the build must make sense.
2. Test it thoroughly. When you think you are done, do it again.
3. Move everything slowly to the new system, think government slowly without rendering her current setup useless.
4. Buy her a NAS with cloud-support at a minimum, or build a second box and setup a backup system that is easy to use but works.
I have had a boss worth well over $100 mill USD that used Windows XP on his main and only computer with everything on it back in 2019 who is a high level real estate owner in a good part of Oslo and i got a company to take care of him instead of putting myself in the crosshairs when he asked me. He is 60+ and computing was totally uninteresting.
He hated all kinds of expenses in a weird way, but he understood when i explained it to him in a manner he understood. All drawings and information about his buildings and basically everything since he started with real-estate in the 80's were on that computer. No backups, spinning rust and i have probably never seen a more worn keyboard.
(But he was also the kind of guy that loved to install stupidly high tech stuff in his buildings and throw dinners for the 2K workers and the companies that were his tenants to show his appreciation, while himself driving a first gen VW E-Golf that had seen better days. His chair in the office was a half broken reject from one of his tenants, and the desk came from IKEA. Probably the coolest guy i have ever met, but thinking about the values involved and even if i could have done it myself, that is not a risk i was willing to make.
Good luck though