With everything out on the market today don't even bother with the mono screens. There are plenty of affordable color models. There are a few models that have special sonar scanning units such as Interphase make(expensive). You standard fishfinder/sounder are all generally the same. Transducers the part in the water are all made by the same OEM manufacturer Airmar. All the major manufacturers use the same general transducers with their connector on the end.
You are correct the higher the power the more expensive and the deeper and more detailed the unit will read. For most people the 600w units are adequate. If you are a more serious angler and have the buck to spend go with a 1kW. Furuno has two new units out the 620 and 585 which are replacements for their 600L and 582L they have always made a good sounder. Garmin has come to the market this year with a quality black box sounder the GSD22 which is compatible with the 292/392/492/3206/3210 it will handle 600w/1kw/2kw transducers. Get in touch with Scuba Steve he can really help you out
www.scubasteve.biz.