VHF is the only one I would consider required. For under $200 you can buy a Horizon HX 890 (Or whatever the model de jour is), which will have GPS and hence speed, compass course, DSC, FM radio, recharge options and is waterproof. I find it very useful when I sail the Flying Scott which has no instruments. That I would call the minimalist option.
The sky is the limit otherwise. I would begin with a chart plotter with inputs for depth (You get water temp with that). You will get SOG, compass course, ETA etc. with also any chart plotter. Wind speed and direction is useful mostly because you won't have to look up at your windex as much. You still will because it is more accurate.
Radar overlay is useful if you operate in poor visibility (Which you shouldn't). You need substantial practice with radar before using it in poor visibility. If offshore, AIS is terrific but I find it almost useless inshore since it creates a lot of screen clutter.
You will never keep up with the technology unless you are really into it. You can have video cameras, infra red cameras for looking underwater, monitors for almost everything, Internet updates (Usually by subscription), sideways and forward looking sonar with satellite overlays and on.
I think it makes sense if buying a system to get all the comments from one company and buy them at the same time. That way they should all integrate to work together and age together. Putting old with new is asking for trouble.
Make sure you can read the displays from the helm. Avoid putting displays on the cabin riser because people sit there and you won't be able to see the displays.
My 2 cents.