If you're simply after a Freeview receiver (not a recorder) then there's very little difference between them, and even the cheapest (eg from Argos or a supermarket) will work effectively. Most Freeview reception problems are caused by the digital signal being weak and/or the aerial being ineffective, rather than because of the box.
Bear in mind, that digital signal strength should be increased when analogue is switched off, so as long as your mum is happy with analogue. it might be best to wait until just before switch-off in your region to buy a Freeview box and test it out.