You cant beat that garmin 7sv if you can find it, I think its been discontinued for a while though.
They will all mark fish, there isnt a bad unit on the market! Just need to decide which features you need and what size screen you want, then look through models and find what looks good to you! Head to a big store like bass pro, cableas, etc and look at them on display. Play with the buttons and see what looks best to you, then go shopping to find the best price on that model. Don't shop for the cheapest unit or settle for a unit because its $50 cheaper.. you lose a ton of resale when you go out that door with it, so be sure you are getting the unit you want even if it costs a tiny bit more. Upgrading can be a pain if you need to rerun transducers and power cords.
I have both lowrance and humminbirds, have friends with garmins. They are all good. I think I like the humminbird helix line the best, but the only new model one that would fit your budget would be the helix 5 si and a 5" screen is too small for side imaging. You can still get the last years helix 7 g2 side imaging for 499.99, I would suggest that. Access Denied (click the access denied link to go to the bps site)