1. Hanhelds - this was answered. It really depends on what you want to do. I don't think existing handhelds have screens sufficient for decent chart work. They are fine as a way to get a lat/lon fix and place it on a paper chart.
2. Garmin is getting tricky in their descriptions. I see most of their units come in 3 configurations:
- no chart
- US coastal chart (or sometimes US inland chart)
- Worldwide "satellite enhanced basemap"
Worldwide basemap is exactly as it sounds - a nice picture with little low level detail.
US coastal chart is pretty much a complete chart showing a full amount of Garmin depth, coastline and navaid information. They differ from Garmin's more expensive charts (sold separately) in that they do not include various bells & whistles like photos, additional information etc). They are fully suitable for navigation.
3. You can buy a very good quality Raymarine ST-40 depth sounder + display for $200 at Defender (and probably even cheaper if you look around). The difference between systems is, as usual, quality. My boat came with Uniden "walmart quality" depth sounder installed by PO. It certainly shows what depth it is - but the display is tiny and film that covers it (internally!) hardened and cracked. A knotmeter (ST-40 series) next to it looks pretty good and is more readable.