I would use the PSP TFT display (480x272 with 24bit RGB) since these are cheap @ $30 USD and readily available.
There is presently only a HDL 68000 CPU core and not a HDL 68020 CPU core.
If you use DDR_SDRAM instead of plain SDRAM, you will get a larger bandwidth and slew rate control in the DDR output drivers. Also, unless you decide to make Amiga specific interfaces, the Amiga schematics may not be of much use to you. You should probably stay with industry standard VGA monitor port, PS2 keyboard/mouse ports, SD card interface, DB-9 Serial port etc....