I have yet to find a weed whacker that is made of plastic that you bump on the ground so that the plastic string comes out that will actually last. My experience is that the weed whackers only last one or two seasons. Manufacturers call this planned obsolescence - consumers call this "man, I got to buy another weed whacker already?".
There are a few places you can find the Craftsman 17 25cc weed whacker manual online. You can also directly request the manual from the Craftsman company.
regular 87 unleaded gas from any gas station. if it is a 4cycle then you will not have to mix gas and oil, only 2 cycle engines require mixing the oil and gas. (this is the same for edgers, weed whacker (weed eaters) any kind of gas powered lawn equipment.
It shouldn't hurt it, it will still burn. Many Semis usually put used motor oil in their gas tanks just to get rid of it as SOP in their motorpools.
Replace it.
one of your lines are broken. really simple to fix.
It is not clear how "average workload" is defined. Keep in mind that a gas-powered weed whacker is more powerful and can work wherever you need. An electric device is limited by the length of the cord or extension cord, which you will no doubt whack through at least three times each season. Weigh the size of the property you want to manicure and decide accordingly.
Replace it
In the rubber part of what?
I removed the gas tank from the weedwhacker and pulled off the old fuel lines with needle nose pliers. Both hoses have something at the end of them that you can re-use on the new fuel lines. Pull the fuel filter off the end carefully. May have to split the line with a razor knife where it has become brittle where it attached to the carb or the fuel filter down inside of the gas tank. Went to Ace Hardware and they sell the fuel line by the foot. That will be more than you'll need. Push the new fuel line through the holes in the gas tank that the original lines came out of and with the needle nose pliers pull the lines up through the fuel filler hole and put the end pieces back on as they were before. Put the lines back into the fuel filler hole & re-install the gas tank on the weedwhacker. Trim the fuel lines to the proper length and re-attach the connections to the carb. May want to replace the spark plug at the same time. Good as new.
First off where is it? If it is in line then you can cut that section off and replace it with rubber fuel line of equal size.
Replace the line, don't screw around with cheap fixes it ain't worth burning your car to the ground.