Entire books with hundreds of pages have been devoted to answering your questions.
It's all in the caliber and fps.
There is no way to give you a brief answer. ONE of my reference books on cartridges is about 2 inches thick, and THAT does not have all of them. Rifle cartridges have been as small as .17 caliber, and as large as .200 caliber. There are thousands of different rifle cartridges.
Up to the 1950s, it was the British Enfield 303 caliber rifle.
22-72
I can say that i shot a fine Antelope buck with my 270Win caliber rifle at 403 paces in Wyoming.I have seen this caliber make many kills from that distance on in to 35 yards.It all depends on the game animal hunted,and most important your rifle skills.
All of them.
Blue Book of Airgun Values
If you mean the jacket, it all depends on the caliber and maker.
Yes, provided you follow all applicable laws and regulations.
Many do, but, not all the time. You will have to call to find out.
It all depends on the model and the condition of the rifle. Re post the queston with the model number and condition of the rifle and I will try to give you a price.
".303 British" is usually used to denote a caliber, as opposed to a specific firearm. Perhaps the most famous rifle in that caliber was the SMLE, which is the abbreviation for Short, Magazine, Lee Enfield. There were several marks and models of British military rifles that originated under that designation, all in that caliber. They were made from prior to WW I, and continued through the Commonwealth nations (and former Commonwealth nations) into about the 1960s. So, yes, the caliber .303 British and the Enfield name are linked- but Enfield is not the full name of the rifles and carbines. Run a Wikipedia search on SMLE for some more reading.