RF won't work for you, and composite most likely won't either unless your TV supports the PAL color system and 50hz. You'd only have a sure thing using RGB or an Indivision.
To use a modern monitor for games, you'd need a scandoubler like the Indivision. Workbench can display on a modern monitor in any of the 31khz modes.
4gb is generally enough unless you want to stick every game on it.
Kickstart 3.0 is fine if you only want to run OS 3.1, I'd go for KS 3.1 to run OS 3.9.
Better deals can be had on accelerators, but Amigakit's might be guaranteed.
