The 280 flash should work just fine but you will not have any TTL/PTTL modes.
The 280 was one of the better older Pentax flash units, as it has bounce and swivel and is relatively compact and has reasonable power.
You might find that using it in the auto mode will work better than PTTL in many situations
This is a great page that has lots of information about Pentax flashes, but most important is the part about "Smart Auto" on Pentax bodies:
jr-worldwi.de: Photography: Technic
If you are concerned about the trigger voltage, you can measure that with a volt meter between the center pin and the ground connection when the flash is fully charged. I do not recall seeing an official statement from Pentax on the safe trigger voltage range for Pentax cameras, but I think that the consensus is to stay under 30 volts or so.
Ray