Replacing the monovalve cartridge will loose maybe a teaspoon of water. The valve body is at a very high point in the cooling system, so barely anything is going to come out if any at all.

Don't drive a 603 with the thermostat out. It isn't an American car and uses a bypass cooling system design. If you like overheating, go for it, otherwise, use a thermostat please.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]