It can definitely be done with webcore (see the FAQ below, that’s a clickable link), you just need to understand that SmartThings does have outages and glitches more than some other systems in a similar price range. So it’s good for convenience notifications, but it’s not something you would use for a normal medical monitoring system.
That said, since in your own case A false alert would mean that you thought your mother hadn’t been in the kitchen when she actually had, I don’t think it’s raising a lot of additional safety issues. Just annoyance ones. So it might well be worth trying.
As far as minimum kit required, you would need
The smartthings hub (any version)
A single motion sensor, if you’re in the US the one sold at Lowes are very popular and generally cost less than $30. If you are in the UK, everything is generally more expensive, but you should be able to get a Z wave motion sensor that will work with SmartThings for around £40.
And since you would probably want to distinguish between your mother being in the house and out of the house, such is at the doctor’s or shopping, it would be helpful if your mother either had a smart phone or if you got the SmartThings arrival sensor ($30) so that the hub would know if she was home or away.
No subscription is required, but the system will only work when the Internet works, so she does have to have Internet at her house.