You don't *need* an expanded Amiga for OS3.9. I have an A2000 with 68000, 2M CHIP, and 4M FAST, and I run OS3.9 on it. Only because I have gotten used to the keyboard shortcuts for the 3.9 Workbench, and it really irritates me going from one Amiga to another and not being able to do certain things.
And, there's no problem with the icons. They don't look as good in 4 colors, but they do work just fine.