A 2006 could have been built a while before and may have steering damper blades. Check underneath for a diagonal spring steel plate between stub axle and chassis, behind the suspension.
If you have damper blades, bin them and fit Mulfab steering bearings. If no damper blades you will have MMC steering bearings with less castor angle
But at that age the bearings could be drying out. So consider either replacing the MMC bearings or fit Mulfab bearings.
Until you sort this out, power steering will only mask the issue, and for a Plus4, isn't really necessary unless you do a lot of low speed wheel twirling.
