power supply

wanted to find out what the minimum wattage required for a configuration running a RAMPS 1.4 variant, 5 stepper motors (nema 17) , extuder heater at 35W, and a heater bead , all running at 12 V

Depends on the power draw of your heated bed. A standard PCB heated bed draws ~10 amps, and the rest draws another ~6 amps. You can get away with a 16 amp power supply, but a 20+ would be better, especially if you are getting a cheap Chinese one as they tend to be over-rated.

thanks a lot, would you recommend any

On this topic, can anyone please confirm that I can run the heated bed off a different power supply from the rest of the electronics? Presumably, using a solid state relay I can.

Also I have read that microstepping works more reliably if I supply the stepper motors with 24 volts.
In this case are the driver boards themselves fed with 24 volts, are they designed to cope with that, or where is the boundary between the 12v supply for the electronics, and the 24v supply for the motors?

In theory, I could make my setup work with 3 different power supplies - some of which I may already possess - rather than shell out fresh money for one high-current one?

Would you consider Dell a good PSU? I switched to one from a cheapo and even though the dell was rated at 16 amps too, it wasn't enough. After using both supplies my, cheapo on heat bed and dell on rest my printer finally stopped skipping steps, started extruding more consistently, and had smoother prints. After that I was able to identify hardware issues and fix them. I now am able to make acceptable quality prints.