Improved pressure so that i can run multiple showers??

Ok, I moved into my home in July. The water is rusty because I have old galvanized pipes. I'm holding off on a washer & Dryer until I get this corrected. Also, I can't take showers if the sprinklers are going, and nor can I have both showers going at the same time, and that is a major inconvenience.

I primarily decided to got with PEX due to the ease of install, and more importantly I like the idea of each fixture having its own line. It takes forever for the bathrooms and kitchen sink to get hot.

I've had a change of heart because of the manifold install in my bulkhead because I would have to open up the whole wall just so I could drill holes in the beams allowing the PEX to get through, and for the fact that I had rodent problems with my AC duct work, and don't want the same with my PEX. It could be disastrous.

If I go with copper, how can I assure that problems I listed in the beginning of this diatribe are corrected? Is it the size of the pipe?

I've lived in nothing but apartments prior to purchasing, and have never had problems with water pressure and multiple showers going.

What do I have to do?

If the water pressure is good from the city or well than the pressure will be good after a repipe. I would go with copper piping due to rodent will sometime eat into the pex causing leaks. Hot water will get to the fixtures faster with the repipe because of increase in flow

Yeah, psi was 120 when I first moved in, and I had to buy a regulator

With the pressure regulator adjusted proper and a repipe with new water service you will be good to go. Depending on the age of the water heater you might want to replace it too.