how deep is this chasis in the rack?

The chassis depth is 17.3 inches excluding the rear fan cage and 23.82 inches including the rear fan cage.

How many internal 2.5" drive bays does the SilverStone RM52 support?

The SilverStone RM52 supports 4 internal 2.5" drive bays.

The maximum GPU length is between 160mm and 190mm, but removing the 3.5" drive cage eliminates the PSU depth limitation.

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SilverStone RM52 Black Steel 5U Rackmount Server Chassis Primary PSU slot: PS2 (ATX) / Mini Redundant Secondary PSU slot: SFX (occupies 3.5" bracket)

Primary PSU slot: PS2 (ATX) / Mini Redundant Secondary PSU slot: SFX (occupies 3.5" bracket)