I've not used the treadmill - I actually road run myself, but if you have any concerns I'd try & find a place locally that sells them & try it out if you think there are any issues. You don't have to buy it at the shop you try it out in, as usually they are much more expensive there. The other alternative is to see how wide the running area is & then mark that out & see how you feeling running on the spot in that area - it's not the same as the real running gait, but at least you'll get some idea.
To purchase you ought to check out fitness superstore as they've sponsored our contests, so it's worth at least comparing them to other places (& if you do buy mention you went to them through us!).
If you're son is after a home gym, most likely he'd actually be after weights I reckon. Benches, barbells, maybe the odd machine (depending on the size of the garage). If he is after a weight room as well as Fitness Superstore you might want to point him to Watson gym equipment which makes top of the line stuff & (more expensive, but very high quality) Pullum Sports. Those are about the best places in the UK for gym stuff ...oh yea & of course The vegan Sports shop which only has a limited range of stuff, but is (obviously) completely vegan (so no worries about leather etc).
Hopefully that is of some help, sorry I couldn't help with the size issue.