Hi,
I have had a good run with Navamn chartplotters over the years. I currently have a 5605. IMO:
A; Navman
B; As big as you can afford (Navman 5605 or 5607 are 6.4" around $1500 local and $1000 for imports)
C; Navman uses c-maps. I have found them very acurate and detailed.
D; Internal is simpler for open boat or canvas canopy. You will need an external aerial for hardtops.
E; You need to stack up all the data; features; user friendliness; reliability; performance; cost and decide what is most important to you.
F; I have found that colour is streets ahead black and white (greyscale) plotters. I would not buy anything but colour.
Cheers Tinman