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.