How much effort you want to go through depends upon how much you value
what you are powering. The cheapest solution is to go to a flea
market a find a power supply.
Dual secondary windings is a good idea, but it's not that much savings
over two transformers. I wouldn't use a non-switching regulator for
the 5VDC, unless you need to warm your house. I'd get a switching
28V->5V 10 to 20W isolated DC/DC converter for the DC portion. But in
any case, this supply is going to cost more that two wall warts would
and, depending upon how skillful you are at building it, it may not be
any safer or more reliable.
I guess a question is, what fraction of a typical machine's value
should you spend on protecting it from its power supply? For rare and
valuable machines, should we be building supplies with dual switching
converters with 6kV isolation, diode mixed with over-voltage and
over-current protection and a soft start circuit? Or is a $50-$100
regulated bench supply for each voltage good enough? I worry about
this every time I turn some machines on....
Eric