Is it possible to 3D print an axial turbine 2 - 4 inches (50 - 100 mm) in radius, capable withstanding temperatures about 800 - 1000°C and rotation speeds of 100 - 120 x 103 rpm?
How expensive is that? Is it cheaper to mill such a turbine from a whole piece of alloy?
What technologies and materials should be used?
Are Inconel alloys suitable for 3D printing?
Are there any titanium alloys suitable for this task? I've read titanium is rarely used in rapidly rotating parts due to its ability to ignite if mechanical failure occurs and rotating blades touch the casing. Do titanium alloys still have this drawback?
Is it possible to make disk of titanium and blades of Inconel, and have them welded (considering heat expansion)?
How blades or blisks can be ceramically coated?
You ask some very interesting questions! Firstly, when researching topics such as this, you will have far more luck using 'additive manufacturing' as a search term rather than '3D printing'. In the professional industrial environment, '3D printing' is not a term that is really used to describe the manufacturing you are talking about.
Selective laser melting is the additive manufacturing process most suited to metallic aerospace parts. Inconel alloys can be processed (e.g. IN718 being one of the easiest) along with titanium (almost exclusively Ti6Al4V). As for manufacturing turbine blades and similar parts, you might find this interesting: Additive Manufacturing - Breakthrough with 3D printed Gas Turbine Blades.
Titanium is not typically used in high-temperature sections of gas turbines, but will be used in larger, cooler components such as fan blades, where it's strength to weigh ratio is a benefit (less mass to rotate = better fuel efficiency).
Coating of high-temperature nickel superalloy components is usually performed with electron beam physical vapor deposition (EBPVD) or thermal spray such as high-velocity oxy-fuel (HVOF); each process has certain characteristics that dictate when/where is it used.
This is only really a surface depth answer to your questions, but it would be impossible to answer fully here!
(My experience: PhD student using selective laser melting with aerospace alloys)