The original C:DMD corebox and the FotU are both standalone, having all the tokens/rules/etc needed to play. The ruleset is essentially the same in both - they are compatible.
The scenarios in the original corebox are 'season 1'.
At the same time as the original KS, a box of additional scenarios came out - that is 'season 2'.
The scenarios in FotU are 'season 3'.
At the same time as the FotU KS, a box of additional scenarios has been created - that is 'season 4'.
It is all compatible, so get either core box (original or FotU) and get the other and seasons 2/4 just for more scenarios to play. (Scenarios sometimes have extra minis in them, and the core box & FotU contain mutually exclusive sets of investigators).
The scenarios in the original corebox are 'season 1'.
At the same time as the original KS, a box of additional scenarios came out - that is 'season 2'.
The scenarios in FotU are 'season 3'.
At the same time as the FotU KS, a box of additional scenarios has been created - that is 'season 4'.
It is all compatible, so get either core box (original or FotU) and get the other and seasons 2/4 just for more scenarios to play. (Scenarios sometimes have extra minis in them, and the core box & FotU contain mutually exclusive sets of investigators).