At the risk of appearing indelicate, pretty much forget checking models.
When it comes to sounders, $500 is penn/banax budget. Even a lowrance hds5 is around $850 and a furuno 620 is more like $1200.
I think at the lower end of the market, furuno doesn't give you good value for money and I'd probably buy lowrance. Personally I think the HDS8 is the best pick, its the cheapest lowrance with soft buttons.