I am far from an expert, but I can help you with some of the questions. I have a Samsung Galaxy Tab that I like a lot, but I certainly wouldn’t call it a replacement for a computer, depending on what you use your computer for.
It’s pretty good for internet viewing, and I suppose that’s mainly what it is marketed for. However, most but not all websites show properly on it. Sometime you will automatically get the mobile site, sometimes not. If you get the mobile site, you can usually view the regular site if you like.
A tablet does not come with Windows. I assume you will be looking at Android tablets since you do not want an ipad. Android is the operating system. You can find apps that allow you to view and edit windows files such as word and excel. I think Microsoft will be making tablets at some point, but right now they only have Windows phones.
As far as I know, you can only run apps, not full fledged programs like Photoshop. Take a look at Google Play or Amazon Appstore to see what kinds of apps you could get. They certainly have simple photo editing programs, but I am not aware of something similar to Photoshop. Check to see if you can find apps that do what you would like your tablet to do.
I think all tablets can be used wifi only, actually some will not be compatible with a cellular data plan.
I believe it is possible to use with a wifi or bluetooth compatible printer, but I haven’t tried it and can’t say for sure. A tablet may not have USB to connect to a printer.
Some will have a fixed storage, such a my Galaxy. Some may allow you to add an SD card.
To be perfectly honest, my tablet is a toy. I check my email, and facebook, listen to music, read ebooks and play games on it. If you need to do actual work, a tablet may not be what you want. If you describe what you hope to use your tablet for, someone may be able to give a better idea if it can really fit your needs.
A friend of mine described tablets in general as designed for consumption, not production. You look and listen to stuff (websites, mp3s, ebooks, etc.) but you don’t use a tablet to create things, either because it’s inconvenient to do, or just not powerful enough. I think this is a pretty good description, and something to think about before you buy one.