Coup:
Do you need GPS with your FF or is that covered?
For sure get DF. The 50khz is very handy. Never a bad idea to pass up redundency when it is cheap. If your down speed probe/system fails, you will like seeing your balls to confirm depths.
I suggest you get the speed and temp sensors as well. As stated above, redundency is great and cheap when upgrading a system. It is always expensive and dear when you find you need it later. Also, you will not want to be using your down probe when fishing the shallows for browns. 10 feet of water is no place to drop your probe. You'll be crushing mollusks before you know what's happening. Also, the speed and temp at 10 feet will be close enough to surface conditions. You'll be looking for surface temp breaks. Also, late summer and fall when targeting salmon in the shallower water, the probe is at risk when following close to the bottom. Lots of fish taken several feet above bottom in 75 FOW. The bottom can rise up and smite thy balls with a mighty and swift vengence.
I bought a Lowrance LMS-527C DF with internal GPS last year. All the probes included for about $675 from Hodges Marine Electronics. I like it very much.
I just upgraded to a larger boat (keeping the 18 footer) and just bought all the sensors and harnesses to plumb the 527 into the new boat (28 footer). I looked at the FCV 620 for the new boat but when push came to shove, the 527 works so well that I couldn't justify spending more money for another FF. $330 for all the sensors, harnesses and brackets allows me to swap out my control head to which ever boat I'm using. (This also lets me learn and stay current on 1 system versus 2 which becomes more important with age (I'm sure glad we don't need to program VCRs to catch fish)).
More importantly, if this is a new-to-you boat, think about your electronics as a system. All the major and good brands have thier electronics worked into networked systems. My new-to-me boat has a menagerie of different manufacturers (SITEX, Northstar, Raymarine, Cannon and now Lowrance). not much networking abilty there. I frequently think of ripping it all out and going with 1 brand so I can multifunction the control heads. Maybe I'll do that if I add RADAR.
Just some thoughts to consider.
Jekyll
Dapper Dan
Mexico