Don't know anything about Garmin's marine stuff as we use Furuno but I will say that a 10" screen is better than a smaller one. We bought a 7" Furuno NavNet VX2 because the 10" version, as much as we preferred it, would not fit on the very clever overhead retractable mount a previous owner had installed. Fortunately this mount places the NavNet very close to the helmsperson so the 7" screen has not proven to be a detriment. But even in this position, the 10" screen would have been nicer.
So regardless of what you might get, I would suggest going with a decent size screen like the 10" on the unit you're contemplating.
One thing to make sure of is that this out-of-production Garmin is still supported with whatever chart cards or chart software it uses. I have read this can be a problem with Garmin and other manufacturers--- once a unit is out of production the data it uses is no longer available. So even for $400 you don't want to end up with a plotter that you can't load the relevant chart data for your area in.
And of course you want to make sure the kind of chart data it uses is one that you like. Our two plotters both use C-Map, which are vector charts. I much prefer this type of chart to a raster chart which is in essence just a picture of a paper chart. But a lot of people prefer raster charts. And some of the newer plotters can display both.
As to having a plotter at the lower helm, since we only run our boat from this station, that's where our plotters and radar display units are. We have a handheld C-Map GPS plotter that we can mount on the flying bridge if we run from up there, but we virtually never do. But if weather or whatever sometimes dictates that you run from the lower helm, then having a plotter down there would be handy I would think.
-- Edited by Marin on Wednesday 13th of January 2010 01:54:04 PM