hriehl, a couple of years ago I was in the same position you are in- I have money to spend and no idea how to spend it.
To answer your questions:
1. handhelds are fine in a small boat, and fine as a redundancy on a big boat. BUT, the screen is smaller than even the smallest fixed mount, so unless you have eagle eyes or the ability to keep it close to hand in a cluttered cockpit, it's not my first choice. Also, because it is handheld, by definition it is going to be easier to lose overboard than a fixed mount.
2. Basemap quality varies from manufacturer to manufacturer- for example, the Lowrance/Eagle unit I have has very good base cartography... for the US. Since I live in Canada, there is no base cartography included, so I had to buy a Navionics card. BUT, that $200 Navionics card also enhanced the US basemap, with more info on marinas, additional depth info, etc. If you are crusing in US waters only, you can get by with just the base map on the Lowrance units- I don't have any experience with garmin.
3. You can buy Chartplotter/depth combos, which is more economical than buying two stand alone units, and you can usually display the depth info three ways, either as a text overlay over the chart screen, or in split screen mode, which will give you two displays on one screen, charts and bottom profile, or as a full screen display, and you can "toggle" between chart display and bottom display.
I still do most of my routing by hand on good old fashioned analog charts first, and throughout a sail I like to practice my dead reckoning skills, BUT, i have found that the chartplotter has improved my sailing skills, because i can get almost realtime feedback and verification of the changes i make in sail trim, crew placement, and it is good for providing "at-a-glance' verification of position, distance travelled, distance to destination, etc.
With the right plug and play inputs, you can slave your autohelm to your plotter. I haven't bothered, but it can be a nice feature for some.