We recently got a Lowrance HDS-7 Carbon at a very very good price because the new ones have just been launched. Does a bloody good job. For your purposes I will observe that the touchscreen freaks the hell out if it gets more than a bit of spray on it; the sensitivity is a lot lower than say, a modern smartphone, probably so that it doesn't do a loop-de-loop with a bit of moisture.
With regards to maps, whatever you get just make sure it does live charting. Lowrance calls it Sonarchart Live or Genesis Live (Navionics vs C-Map cards), can't quite remember what Garmin call it but they definitely support it. Within a couple of trips to your usual spots this turns a cruddy base map into a ridiculously high-definition bathymetric chart.
I haven't had the Lowrance out past about 32m yet but I'm sure 600w is plenty for it. I regularly fish from a boat running a Furuno with a 600w P66 and it's good out to a couple of hundred metres, don't expect to be able to see a fish eating your bait at those depths though.
Although I've only played with it a little bit I'm really liking the Lowrance's "Fish Reveal" thing, or more correctly DownScan overlay. This uses the 800khz bottom imaging, which gives fantastic seafloor/structure views (but is up to **** all for showing fish a lot of the time) and overlays it over the High Chirp (or whatever sounder mode you prefer) image of the water column, which is better at showing fish. Superb fish-locating abilities in the up-to-30m I've tried it in so far, and I feel like if I lower the frequencies it will keep working for a fair bit more depth.