While purchasing a DSLR which factors should be kept in mind if your aim is to zoom your photograph to 100%, frame it, and hang it on a wall?
At 100% there shouldn't be ANY noise.
Noise is like death and taxes, it is unavoidable.
Even the most expensive cameras produce noise and, although, it may only be visible at 100%. The base ISO, usually between 100 and 200 is almost noise-free, but you will still see noise in images, particularly in shadow areas.
What strikes me as odd about your request is that 100% scale can give you very different print sizes depending on the camera and print resolution. If you print an image at 300 DPI from a 8 MP DSLR, you will give a MUCH smaller print than an image from a 24 MP DSLR printed at 180 DPI.
What happens to noise is that it gets averaged-out and becomes visibly diminished when images are scaled down, so if you have extra pixels and can afford to scale down for the print size you want, you will be better off in terms of noise.
There will always be noise - but you can minimize noise, you can remove it in post processing or you can understand that in most cases noise doesn't matter.
To minimize noise you need lots of light, there will always be noise but if you have a bright image the image will be so much "stronger" than the noise you wont be able to notice it - so, the minimize noise you have to over expose your image just a bit (but not so much you blow the highlights) and avoid dark areas.
To do so turn on the histogram display on your camera and expose the picture so that the all the histogram data is on the right side (but without hitting the edge).
Of course, nothing in life is free - if you do it too much you will get low-contrast washes out images too little and you get noise
What's better, a boring washed out noiseless image or an interesting image with some noise?
Noise reduction in post processing
Every software that is designed to handle photos has a noise reduction feature - some are better than others but they all basically work by hiding small details and blurring the picture a bit, the stronger the noise reduction the more blur you get.
What's better, a blurred photo without noise or a crisp photo with some noise?
Understand that noise is fine
Normal people (that is, not photographers or graphic designers) will never look at a 100% crop of your photos, also, the bigger you print the farther away from the picture they will go to look at it - so they will never see the noise.
Normal people also don't notice noise, unless you have extreme noise people just won't notice it - they look at your subject, expressions on faces (if any), they notice color and contrast but their brain is ignoring the noise and they just won't see it unless someone points it out for them (unless the noise is so extreme it's hiding important details of the photo).
What's better, a photo people like when they look at it or a picture that is technically perfect in ways your audience will never notice?
Bright images have less noise, if you have little bit of noise you can remove it in post processing and, most importantly, unless you are shooting stock photos or most of your viewers are members of camera clubs (or other types of "pixel peepers") some noise doesn't matter at all.
I don't understand what you mean by zoom your image to 100%.
Do you mean print it at whatever size allows you to print at 300 dpi (assuming that's your printer's native output?)
Looking at your photo at 50% on screen is a good way of approximating what it will look like when printed.
At 100% there shouldn't be ANY noise
This is a poor assumption. The world of film cameras (and pre-digital movies) had tons of 'noise' due to the way film is constructed and processed. A grainy picture can often be better than a smooth one.
Texture is important in photographs and part of the texture is the 'noise', a texture free photograph would actually be more like a cartoon or those horrendously overdone 'airbrushed' covers on magazines.