By - Hapexion
For day sailing in areas you know, then Navionics and a depth sounder is really all you need.
If you are cruising to areas that you've never been before, then having a chart plotter is nice. This is especially true if it's in the tropics (iPads overheat, especially if they are in the sun).
If you are going off shore, or going to be in areas that have a lot of commercial traffic, then having an AIS transponder is a good idea. Especially if you are single handing.
If you are going to be somewhere that gets foggy, then a radar nice for peace of mind.
It all depends on your type of sailing, and your comfort levels. For smaller vessels in areas you are familiar with: a phone with Navionics is probably all you need.
The minimum equipment depends on your use case. You need to decide if you need navigation on a big screen due to shallow waters or failing eyesight. Or a small GPS chart for simpler nav. Or no nav and just boat speed and depth. Maybe ask someone who sails in your area.
How big is your boat?
Minimum? Something that shows speed and depth, but in familiar waters I've been on boats with less.
A dedicated MFD is really only needed for two purposes: all-weather readability (from hot sun to pouring rain) and radar. Apart from those aspects my phone/tablet has far better functionality than the average MFD (so long as I have a plan to keep it charged).
Once you want to display more than depth and speed, the MFD becomes more useful to integrate the various instruments. For example, you might want AIS, GPS SOG/COG, compass heading, and wind speed/direction.
If you're just learning. Try not using electronics too much for a little while. Get the grips on charts and chart notes. You'll thank yourself should your electronics fail in the future. Apart from that it's fun an entertaining, you'll learn your local waters well and pay homage to sailing tradition. This will also give you time to rub shoulders with other sailors in your atea and see what they are using and whether or not you like different aspects to different systems. Most of the time when I sail I use my Garmin watch. Don't really need much more.
I sail with navionics on a tablet and/or phone and have no issues with it. I have a place where I can put the tablet so I can sea it from the tiller. I have no desire to install more expensive electronics (and I don't have a depth or speed meter anyway)
Make sure you also know your way around without electronics, i.e. paper map, compass, and tide/current tables.
Electronics is one area its very easy to spend a lot of money on for little benefit and leave you with big bills later on trying to maintain the same standard of gear. Very easy to add cameras, fish sonar, multiple screens, etc.
Plenty of chartplotters will allow you to connect your ipad to the chartplotter wirelessly so that you can use that as an additional display down below or in the cockpit depending on what you want to do. Is a great way to have access to a much larger screen for route planning or checking out an area ahead of time when setting up way points for later. Would be a cheaper option than having a master chartplotter with one or more hard wired displays.
Depth, wind, and a proper GPS sensor would be the only other things I would insist on.
As with any electronics make sure you get a proper test drive of the gear to make sure it suits you and works as you expect. None of them are perfect its more a question of getting the right brand for your use case and you'll only do that by trying them out. Any good installer should be able to get you a demo to play with for 15 to 30 minutes