It is a ScotRail service.
The traction varies from day to day, so it is impossible to tell what it will be. If your service is direct, it will be either a class 170, or a HST. If it involves a change at Perth, you could have a class 158 or 170 from Kirkcaldy to Perth, and then a class 170 or HST from Perth to Inverness.
For first class, the quality depends on the traction. First class on a HST is miles better than that on a 170. There is no first class on a 158. However, 170s have fewer seats overall, so you may want to go first class to make sure that your mum gets a seat. I have never done ScotRail first class, although I do travel semi-regularly on the Edinburgh/Glasgow to Inverness route.