Hi magie,
I hope I am going to be of help here, rather than be a doom & gloom merchant, so here goes
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1. If for example - an image would be changed in 80 % of the artwork = that is OK
Unfortunately untrue. If the new work is based on an original work or if it includes elements of the original which are involved in the make up of the new work however placed, then copyright / intellectual property rights will be seen to breached. Unless copyright has expired of course.
You could however reproduce the 'style' of art as long as you're not selling it with a Cieslewicz signature on it.
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2. If the illustrations were produced in a magazine – to support the articles and if you purchased such magazine it is your property, which you can use it as one pleases – is this Wright?
Works may be reproduced for the purpose of review, critique ,education etc or as you say to support an article but copyright of those images still remains the property of the author (artist) and permission should be sought before publication as he may put forward a moral obligation requirement for the use of his work. Although they may be reproduced for the above this does not give you rights over those images.
The magazine does indeed become your property, but its content does not. It may not be reproduced without prior permission from the author.
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3. So if I ma organising exhibition and want to base it on past artist works/illustrations that would go under QUOTE - 1. the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;?
Have a look at some examples of the magazines but I think I would need to preserve them in some way like laminating or something else – need to also find out more about preservation of paper prints!
Again, there are moral obligations usually involved in the showing of works so permission should be sought before any exhibition of copyrighted images but if the exhibition is for educational purposes there shouldn't be a problem.
If you have original magazines do not laminate them. This automatically renders them worthless, and the best way to protect them is with a tissue paper wrapping and stored flat in an archivists file. Do a google search on correct storage and preservation techniques.
I may be going slightly over the top, but having been stung before with someone reproducing my work (not very well either) I know what it feels like to have your property used without permission.
Do check out the DACS website though and another good one is Artquest.org.uk. They are full of info, and another for you is intellectual-property.gov.uk which is very useful. Bear in mind though that law is different for other countries and the above is only relevant to the UK.
As I said, I hope this helps.
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It's disgusting! 
Did you find a way to protect your rights?
