50 VA (50 watts) is not hard to achieve.
Is it that you want to learn about designing a switching supply, or just want to have a supply with these output voltages? If you just want a supply, then you could pick up a bench supply at a surplus electronics place, probably for not much money at all. Or, you might be able to just cut the connectors off an ordinary PC power supply.
If what you want is to get experience designing a SMPS, then I'd suggest going for a single output voltage first. There are a lot of nuances in SMPS's to learn, first. Parasitics matter. Diodes and transistors aren't ideal, so you need snubbers in various places.
In a modern switcher, the main switch for a supply of your wattage will generally be an N-channel MOSFET. If you look at the application notes for a PWM chip meant for this application, there will be help in specifying the transistor. An example of a power supply PWM controller would be the National LM5020-1. Offhand, for your application, I'd say you want a buck converter, and the transistor will need to tolerate a drain-source voltage of 400 volts (rectified line voltage in the US = about 170 volts), and be rated for a peak drain current of 5A. You can find this kind of transistor for a few dollars at a place like Digikey.
The feedback circuit depends on what controller IC you're using. It may be as simple as a resistive divider across the output, wired to the feedback pin of the IC.