Your 78yr. mother is asking alot for a typical system. A desktop will do but most are that desktop and aren't meant to be moved much. The laptop would still be a good choice as an ext. monitor can be attached that's much larger than the laptop and of course some laptop are 17in. too if going that route. Having Excel and Powerpoint while demanding for any system, will work on laptops too. The purpose of any mini is to use less desktop space but their design also may lack of possible expansion up to a point. If anything visit vendor websites to see what they offer and review the specs to include the weight and/or space requirements.
I'm looking to buy a new PC for my 78 year old mother. She does email and plays games like solitaire and bridge. She also uses Word and Excel and plays a bit with Powerpoint. Therefore, she doesn't need anything super powerful. She complains her current PC tower is too heavy to move when she needs to take it in for repairs. I had planned to find a laptop for her which would be easier to move. But now a friend has suggested a mini-tower which would allow her to have a larger monitor (something she wants). Do mini towers work OK? Any suggestions on brands/models I should look into?