Personally I wouldn't use wifi for anything mission critical. But I'm pretty conservative on that front, and want nav systems to work correctly all the time, every time.
I share your approach and desire to maintain dedicated nav systems, with integration only via industry standard interfaces. Where this becomes a challenge is with radar and fish finders since there are no industry standards for communicating fish finder or radar echo returns. This leaves you with a handful of options:
1) Install a dedicated radar, and interface it to CE and you other electronics. All your sensor data gets fed into the radar (including AIS) and the radar's target tracking info gets sent to CE and plotted do you see ARPA targets on your charts. I think this is the preferred approach if you can make it work. You get the best quality radar with dedicated controls making adjustments and operation super easy. The down side is that it can take up more physical space, i.e. a display screen, than many people can devote to the task. It's generally no more expensive than an MFD+Radar serving the same purpose, but if you already have the MFD then the incremental cost is more than just adding a radar scanner to an existing MFD. In your case I expect the cost will be a wash since you don't have an existing MFD. The other down side is that I think Furuno is the only mainstream company that still makes dedicated radars. Simrad has one, but it's just one of their MFDs turned on it's side with functionality limited to radar. Until they fix their ARPA issues which have been unaddressed for years, I wouldn't go near them for radar.
The up side is that in my experience Furuno makes the best radars so it's who I'd want in the first place. Take a look at their web site and you will get a good sense of what's available. It sounds like a dome radar is fine for your use, and they have a number of models, with and without ARPA. I have an 1835 with is about a 12" screen, 4kw dome. It's a great little radar, and integrates nicely with CE. Cost is about $3500 without ARPA, and $4000 with ARPA.
2) Create a pseudo-dedicated radar by getting an MFD+Radar. But only use the MFD to control the radar. Cost will probably be about the same as a similar sized dedicated radar. I think the down side is that you give up the dedicated radar controls and need to poke through menus and pop-ups to control things. And you will need to check very carefully to be sure the radar transmits target tracking info that CE can use. Although I think this is a viable approach, I didn't pursue it because it compromised too many radar features. Most MFDs didn't have the full set of radar features found in the dedicated units.
3) It's probably a higher-end solution than what you want, but furuno's FAR radars can be interfaced to CE via Rose Points Radar Interface. This will overlay the radar echo returns on CE. I haven't used it, but it's another approach is you really insist on radar overlay. Personally I find radar overlay makes it harder to pick out important chart features because of radar clutter, and harder to pick out radar returns because of chart clutter. I much prefer just display radar target tracking icons on CE so I only overlay the things that I want to keep a close eye on. The down side of this approach is cost. The FAR radars have come down a bunch in price, but are still pretty expensive, and the radar interface is an added cost too.
