With the new chaos codex, pure Nurgle armies struggle a bit at 500 points. It's no longer combat patroll so you have to have the HQ and 2 troop choices leaving little flexibility. Luck would have it that plague marines are in my opinion the best unit in the game so don't be sad that you have to spend a large proportion of your points on them. I spend 75.7% on 5x7 man plague marine squads in a 1500 point list.
Few people play 500 point games that often as the lists can be a bit unbalanced (I have seen 3 basilisks in a 500 point IG list) but they provide something to do as you collect the core of a larger list.
HQ: I would stronbly recomend a Daemon Prince over a chaos Lord. MoN on a prince makes him almost bolter proof and the common S4 power weapon becomes pathetic against him. Wings are great but in a small list you can do without.
Troops: Standard Nurgle doctrine usually goes for 7 man squads with 2 meltas or plasmas and a champion with powerfist. You can't afford 2 such units but I would try and get one squad with 2 plasma and no champion and make the other squad have 1 melta and a powerfist champion.
