northstar is exactly the same as navman, OEM'd through a different company instead of plastimo
I had a mono navman 5500 on the last boat, there's still afew around at the £160 mark, easy to use and was dead reliable but the garmin 182 have come down in price now, bigger screen, full colour and most ar under 200 quid
with either you can buy a chart cartridge at a later date if need be