This is your Handheld VHF: Clicky
(might still be a RIBnet discount there?)
A Chartplotter will not give your actual depth, just your charted depth.
For Actual depth, you will need a sounder/fishfinder or a combo plotter.
Your budget precludes a combo. If I were you, I'd go for a second hand plotter and a cheapo sounder (Cuda) - the most basic rig that reads the depth. Your budget is way blown, BTW
As Bogman says, a sounder gives you historical data, it tells you what you didn't hit. If you are in tricky territory - you will have to proceed at displacement speeds and use the MI eyeball (or paddle) for depth judgement calls.