If you already have a sonar/fishfinder - you could get a separate chartplotter. I think separate is definitely better. I have a small garmin 400C sonar, low price about $450, that's ok.
And a Garmin handheld gps Montana 600. It is excellent. 4" touch screen that is clear and readable in all light. The Garmin chartplotting works very well I think. It can be removed and carried easily, for outdoors use, and for auto use in a vehicle mount. In the boat it sits in a marine mount, connected to the boat 12v, so it stays on all the time without using its battery.
It has one major advantage over specialized marine chartplotters - having a magnetic compass. Without experience of a chartplotter, the significance of that probably wont be apparent. Basically, it enables the boat icon on the chart, to show exactly up to date in the correct direction - which helps a lot in orienting positions of other things, particularly rocks!
Its the top Garmin hand-held, not cheap, about $1000 in NZ - I bought it from US online - cost in $NZ
Montana incl delivery by Youshop $493
chart NI from PBtech $143
marine mount PBtech $58
Total $694