If it were my money and knowing the consoul space available to you i'd go for a combined unit, its likely to get very wet so i'd suggest your remit be completely water proof, Garmins are not completely immersion proof !!
A fish finder/echo is a great investment especially where i operate as its always shallow, dont underestimate your local knowledge though as its always valuable, the GPS map makes me lazy and i've started to rely on it even though i can read a chart and plot a course using a compass and timmings/speed over ground..... but what a great bit of kit.....my C Maps even worked right up the Thames last weekend :-)
May have come across an A frame that would fit your boat too, it was made for a valiant but didn't fit, i'd need to get the measurments and height but it looks the dogs nuts mate 40mm stainless steel and very shinny, not sure what sort of money but it wont be a lot as at the moment its scrap !!
Hey all this sensible talking today has to have earned me a few credability points