Help selecting a Mosfet/driver for 300vdc 3amp continuous on Arduino
Your safest bet will probably be an isolated SSR. I would not recommend building your own switching circuit for that kind of voltage/power if you can avoid it.

I only googled for a minute, but it looks like you've got plenty of options, for example: http://www.mouser.com/catalog/645/usd/1929.pdf

These SSRs don't draw much power so you should be able to drive them directly from the Arduino pins. If you're sure your load is resistive, then you won't need any other protective components.

If the device you're controlling needs safety cutoffs, I would recommend setting that up with a second SSR or relay, so that any one device failing can't result in your load getting stuck on.