It is possible but unlikely that a 250W supply will do the job.
Not because 250W would be insufficient (although it may be) but because it is likely that there will be insufficient current available on the 12v rail from the PSU.
The first link below is very useful - better (IMHO) than some other PSU calculators, because it deals with the 12v requirement
in addition to the overall requirements...

I use Seasonic myself - I've had some trouble with budget brands in the past - but the 460W Gigabyte model from Scan looks a bargain at £25.84...