motherboards for all 500s are teh same. you can swap around CPU modules as you like.

all screens are also interchangeable (dunno about the 550c but at least all 520, 520c, 540 and 540c)

inverters and display boards are the same for all displays.

you can as well try to overclock your 520s CPU to 33mhz and see how well it runs if you want to save money.

putting all this effort and money into a 520 seems kinda useless to me. why not just getting a 540c?