At the lower budget, second-hand range, I love my Garmin Pilot III. You can update the database for £20 or so whenever you feel you need to, it has a good battery life and a decent screen with moving map, HSI etc.
Whatever you get, an external antenna is probably a good idea especially if you want to use it in a spamcan. I'm a very satisfied customer of
www.gps-antenna.co.uk (get the "active amplifier antenna" for about £12 inc postage, and you can remove the magnets pretty easily to avoid interfering with the compass)