Welcome to the site, MarkusA!
It's a tricky one. For a start it's unlikely that you'll ever get the books in first edition and the same format. A first edition of The Colour of Magic, for example, is a completely different format to the other books.
The paperbacks come with the Josh Kirby covers and you can get a nice collection of those, but then Paul Kidby took over the covers after Josh died and the format changed. And now, with Unseen Academicals, the format has changed again.
A lot of people collect the so-called 'black' covers, and you can get a nice collection of those, except that Eric was published by a different company and they don't do a 'black' version.
A first edition of Colour of Magic is likely to cost you a lot of money. There is a facsimile version available, but it depends on how much of a purist you are.
And then there's the Discworld 'Unseen Library' editions, which are very handsome and would look great on any bookshelf (see example here
) but they only ever issued up to Maskerade (I think) and then the company went bust.
So really, it's up to you.
"Goodness is about what you do. Not what you pray to."