Coincidentally, I've only very recently ventured on the same path; getting back into 40K after a hiatus of nearly a decade. But in my case it's Kill Team that reeled me back in.
As luck would have it though, I was gifted the Dark Imperium box for my birthday back when it came out, but it had been sitting undisturbed ever since.
I went for Death Guard too though, and the DG miniatures in the box were a good start. But I needed more variation, like a heavy weapon, so I got the push-fit 3 marines sprue for cheap on Ebay.
This meant I now had 10 regular marines, plus a bunch of characters, 20 Poxwalkers and a Blight Drone. And it began to gnaw on me that I had the basics of a complete 40K army right there.
So I got to thinking; I also already had the Index books that came with the game book, so I basically have everything I need for 40K!
Last weekend, I bought the Death Guard multipart box, giving me another 7 marines, and the Plague Brothers special edition 3 marine set, which I stumbled upon on Ebay for (relatively
) cheap, so I picked those up too.
I'm now at 20 marines; just 1 short of 3 squads of 7 (it'll have to be 7 of course).
Which leaves me thinking of still getting the following: 1 more marine, another Blight Drone (Easy build version, to be converted), and probably two of those Myoptic Crawlers (or whatever the name is
). Then maybe one of those Griffon-like vehicles. For sure I'll get some easy to build Poxwalkers and some characters to buff up the squads.
So basically; with relatively few purchases, you can get a complete Death Guard army.
I'd advise to do what pappa Midnight suggests; get the DG portion of the Dark Imperium box for cheap off Ebay and use that as your starting point; it's a solid base. Of course only if you already have the rules etc.
If not, you might want to get the box, and flog off the Primaris portion to recoup some of the investment.
Welcome to the sickness, and gesundheid