I gave this some thought overnight.
I've been running Solus Budgie, out-of-the-box stock with no modifications, default settings, for a long time without a glitch. I agree with brent that "problems often occur because of theming/icon/widget and customization conflicts", because that is what I see reading the forum. The Solus team is very good about making sure that updates to the Solus core -- that is, the stock build -- are carefully curated and install properly.
I think that failing to update is a high risk, particularly because many of the updates are security-related. That is doubly true in the case of Firefox, which updates frequently with necessary security updates. If your mother is browsing the web, she needs those security updates.
On balance, I would opt for a stock Budgie build, no modifications, and updates.
This thought will probably get me beaten around the head and ears, but have you considered a Chromebook for your mother? It sounds like her needs are very basic, and (as much as I don't like Google and want nothing to do with it), a Chromebook might fit her needs better than Linux.
