Bad blower fan and/or relay questions

[iluv2sled]

I have an 05 xc90. The blower fan has been making noise for about 6 months now. Recently the fan hasn't been coming on when I start the car. If I set the fan control to the highest setting, the fan will start on low and then over the course of the 30 minutes or so, it will gradually speed up to full speed. Once that occurs, it will respond to different fan speeds as I would expect. If I turn off the vehicle, it will start on low again.

From the reading I've done on this and other forums, this appears to be a fan and/or a relay issue, but before tearing into it, I'd like to be sure. Prior to tearing it apart, is there any way to validate that this is in fact the case?

I'm relatively new to Volvos, and I am not familiar with the reputation of the various parts suppliers. My second question, is where does it make sense to purchase these parts? If I go to Volvo, a relay and fan seem to run about $400. iF I go to my local car quest or napa, I'm looking at about $300 in parts. And if I order online, I see parts in the $150 range. We're likely to keep the vehicle for another 3 or so years. For the dollars spent, what makes the most sense?