Fire doors are of extra thickness. They are usually made of a dense compressed chipboard with a paintable or veneered surface. Because the core material is factory-made, it can be tested and certified for fire resistance.
FD30 is the usual grade, it means that under the certified conditions, it withstands fire for 30 minutes. They are 44mm thick and very heavy.
You can buy a FD30 for less than £100, the cheapest ones will be a paint grade with flat surfaces. You can pay more for moulded ones resembling a panelled door, more still if it is veneered. Here are some examples
The door has to be fitted into a compatible lining ("frame") which is not expensive. Chippies prefer to fit a new door and lining together and it avoids arguments about fit and squareness.
The door in your house, being internal, will probably be 30" wide. If a large and expensive house, it might be 32"; if the reverse it might be 28" Doors in several sizes are still made as standard, but they are now stated in metric ( 762mm, 838mm, 686mm, 726mm, 826mm) which you will find are inch sizes really.
You will need certified hinges, these are mostly sold in polished or satin stainless or PVD Brass Plated on Stainless. I especially recommend lift-off hinges for fire doors because they are so heavy.
If you insist on cutting holes in it (not recommended), e.g. for a letterbox, you need fire-resistant fittings. Fire resistant glass is so expensive that you need to sit down before looking at it..
A well-recommended local carpenter (or joiner) will fit a door faster and much better than any handyman, DIYer or typical builder.
The lining can be sealed to the wall with fire-resistant expanding foam, and intumescent strip with furry smoke (and draught) strip fits in a groove in the lining. You can see these and the hinges on IronmongeryDirect.
These doors and the furry strip, being so heavy and solid, are very good for blotting out noise.