Thanks for all the great advices.
Our house warming party happened last Sunday and everyone was very happy. Because our party started at 3pm, we finally gave up the potluck idea. It was also because too many people and relative small space to put their food if they bring large dishes. We had catering service from a local restaurant. Fortunately, most guests like their food and all food costs less than $300 for more than 30 people :) We had a lot of left over, so some friends packed to-go boxes with them at the end of the party. We also had just enough food left for us for this week so no grocery store this week!
Our friends did bring snacks, fruits, dessert, beverage and alcohol, which is really very helpful.
Although we said no gift in our invitation, we did receive a few gift cards with total of a few hundred dollars, more than enough to cover our cost:) (And these gift cards are from local grocery stores or visa gift cards so definitely will be useful, unlike Macy's which I have no idea what I need to buy there....). I'm really very grateful.
Next time, if we are going to have smaller party (10 ppl), I'll definitely do potluck!