It's tough to limit yourself to one god for a strong list. You can make fun lists sure, but they're not going to be super competitive.
For Daemons, good Nurgle choices are:
1. Daemon Prince of Nurgle for your
HQ
2. Plaguebearers and/or Nurglings for troops choices
3. Plague Drones, often accompanied by a herald with the "Grimoire of True Names"
4. Soul Grinders of Nurgle with Phlgem Bombardment.
For Chaos Space Marines:
1. Chaos Lord with mark of Nurgle on a Bike with Powerfist/Lightning Claw. Other upgrades to flavor.
2. Plague Marines with Meltaguns/Plasma Guns in a rhino
3. Chaos Spawn with Mark of Nurgle to accompany the biker lord
4. Chaos Bikers with Mark of Nurgle
5. Maulerfiends (not truly nurgle, but they're not aligned with any god) Paint them green
6. Obliterators with Mark of Nurgle
There's a million possibilities and builds, so I'd suggest you do some more research into what you want.
Most of the time it's best to start with 1
HQ choice and 2 troops. If you go Daemons I'd probably get a Daemon Prince and then a box of Plaguebearers and a Box of Nurglings. This will give you almost a 500 point army.
Automatically Appended Next Post:
I almost forgot, there's a nice new box set out for Daemons of Nurgle.
https://www.games-workshop.com/en-US/Start-Collecting-Daemons-of-Nurgle
Pick that up and a Daemon Prince of Nurgle and you can probably get to almost 750+ points depending on upgrades. It's a pretty good deal.