sonar/gps combo questions

[Turkeyflacx2]

I'm looking at upgrading my fishfinder that came on my recently purchased Fisher to a sonar/gps combo but cant get a straight answer from the salesmen at the local places.I plan on fishing no deeper than maybe 150ft for lakers and maybe a rare trip to the ocean.Id like to keep the price no more than 700.00 too.The queations I have are what watt signal do I need for a good sharp picture.ONe guy told me 1500 would do it and another said 4000.I dont understand the duel frequency tranducer deal (50/200hrz?).And the other thing is with the gps.Will a model with an internal antenaa work through my canvas top or do I have to have one with an external antenaa?The models I seem to like are the Lowrance 525 and the 527.Same thing but one is internal and ones external.Any help would be great especially if anyone has these specific models.
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