Okay, fair enough- that other thread basically only covered decal paper, which you specifically did not ask about.
So, will cover items two and three.
You need a computer and inkjet printer for inkjet decals- I'll let someone else cover Alps printing, which is becoming rarer and rarer. Almost any inkjet printer can be used. You need some sort of graphics software. If you are using Windows, a simple paint program comes with the OS, which is basic but can be used for simple decals. Regular paint programs do not resize very precisely, but it can be done pretty well with some trial and error. CAD programs allow precise resizing, and rescaling by scale (paint programs do too, manually, but with use of some math). Cheaper CAD programs may have less colors, and fewer fonts, also fewer graphics tricks like drop shadows, gradient fills, etc.
Adobe photoshop elements is a popular graphics program with decal makers- I use Paint Shop Pro myself. Both still under a hundred bucks.
I have scanned and resized decals to a new size several times.
One big gotcha is that inkjet decals work fine for darker colors. If the decals have white or very light tones, and are to be applied over dark colors, you have a problem. You must then use white decal paper. This means you must cut out the decals precisely, or you will get white borders around them. I have had some success with setting background color in the graphics program to the paint color the decals will be applied over. This works well with black, but with other dark colors you may have a color matching problem. Computer systems can be color calibrated, but it is a lot of work, and some expense. I have not calibrated mine, and have a lot of problems matching common paint colors other than black and yellow.