I don't have a 257 but I have rerigged my 265 with Garmin. Do the same kind of fishing in NJ.
The 257 has a lot of room for big screens. IMO one big screen (12" +) is better than two smaller screens. You have the ability to split screen but also can view a chart Full screen when you need to. The 8600 series is top of line but the 1243xsv is nice for the price. I have one and I really like it.
The Yamaha CL7 screen is actually a Garmin 7400. You can also use Garmin MFDs to display the engine data/gauges. If you do opt for two MFDs you might be able to save by not getting the CL7. However, IMO its nice to have dedicated engine screen rather than doubling as sonar or radar.
Sonar. You need to decide on thruhull vs transom mount. You are going to want to utilize the sidevu sonar. I switched from transom to thru-hull on the centerline and its a great improvement. GT51M-TH transducer is what you want for the fishing you described.
The Fantom radars just got updated to 50W , so the 40W versions are discounted (while the supply lasts). The 18" radome is good enough. The 24" will give better target separation and marginally better range. The older xHD radars are discounted too. Depends on your budget. IMO if you have a brand new boat, get a Fantom model. I have a 40W 24" Fantom and I really like the motion scope feature and the MARPA tracking.
GET AUTOPILOT.
You cannot live without it. Period.
Garmin Reactor 40 with smart pump . The garmin MFDs will operate the autopilot but I opted for a GHC20 as a dedicated AP control/screen.
Gee, this is fun fitting out a new boat....