"It just felt a little bit ‘slow’ if that makes sense? Like you had to touch the keys quite firm and a little bit of a delay and muted sound."
This doesn't sound good to me especially if a learner is using the piano. However perhaps a tuner technician can put it right but I can only advise as a pianist.
I'm sure in its day it was a fine instrument, however the hammers might be getting hard and it certainly sounds like the action will require a fair bit of work to get it play evenly. The heater approach could go a couple of ways in so much as drying it out too rapidly if it's suffering dampness. I've seen the hammer felts detach under such conditions on similar pianos and the sound boards crack as the drying occurs.
Fair enough if it dries out and plays well, you'll have something to work with. On the other hand, if you look for something from a dealer it'll have some kind of guarantee and once it's in your home and settled if there are any issues they will be sorted if you invest in something from a dealer who stands by what they sell. Also you could re sell if the learning does not become a long term pursuit.
I've been in a position of having a daughter who wants to learn, and the advice I was given some years ago was try and provide a playing experience which will actually sound like it is supposed to when something is played correctly. Nothing more disconcerting than spending many hours learning and it still doesn't sound like the examples the student might reference to compare their own playing. My daughter kept at it and at age 12 she's now working on her grade eight.
As a parent I fully understand the strain of finding funds for all things music and have had to juggle a number of times to provide decent instruments. When one adds up the cost of music, tuition and hours spent learning, the cost of the instrument doesn't look so bad.
I hope the piano works out, if it doesn't you might be surprised how many pianos there are out there which don't cost the earth and actually play well enough to have a decent go at finding out if it's going to be a long term interest. Nothing worse than having a great lump of metal and wood sat in the house which is not worth selling and then having to dispose of it.
Best of luck.