I'm not a realtor, so I can talk freely about most of these areas.
Merrick is a great area, very residential, great schools - just be prepared to pay the taxes that go with this combination. Bellmore is a nice area just like Merrick is. You can't go too wrong with either place.
East Meadow is pretty good too, especially if you look in Barnum Woods.
Seaford? I grew up in Seaford - I don't recommend Seaford at ALL. NO commercial tax base, it's not a friendly area, and it's way too homogenous in every way (conservative politics, race, and religion).
Westbury is touchy - parts of it are really nice and more expensive, but other parts are close to New Cassel.
Levittown will have a lot of capes with no basements. The schools are pretty good - try to go for Island Trees if you can, because the taxes are about $1000 less. For 500K you can get a great home in Levittown.
Hicksville has great low taxes (with all of the commercial activity) but again, the high school is a little rough, and you have to be careful about which part of Hicksville you're in. Some of Hicksville is close to Plainview and Syosset, and it's a nice area. Some of Hicksville though, is in a not-as-good area.
With up to 500K, you do have a lot of options. Almost too many lol!