Yes weather could be an issue. March is very changeable. In 2010 there was a low of -2 C but the record low for that date is -20 C (2009). You also have to travel over at least one high pass if you use either the Coquahalla or Highway 3.
The potential is always there for a storm, and so you need to be prepared to react and respond to changing weather conditions, including spending a night somewhere that you may not have anticipated. There are a couple of high passes you have to drive over to get to Kelowna that are notorious for bad weather, and while storms in March are rare, they are not unheard of.
The thing to remember is that storms usually blow through in a few hours, and then you need a few more hours to let the crews do their work and clear the road of the snow, and so you need to be prepared to just stay put for 8 - 12 hours and wait that out.
Have you looked at flying to Kelowna instead?
Are you planning to travel the Coquihalla Hwy 5 or Hwy 3 to Osoyoos then north to Kelowna?
The Coke is one long climb up to the summit (~1200 m) and one long slide down the other side. Hwy 3 will have Allison Pass (~1300 m) and Sunday Summit (~1300 m). In early March, there would quite likely be some snow falling on those highways, especially at the higher elevations.
If you can get dedicated snow tires, the ones with the snowflake inside a mountain logo on the sidewalls, it would help.
I often go to Penticton. I loath winter driving on the Coq. My suggestion would be to take Highway 3 to Keremeos, take the Keremeos bypass and then on to Penticton and Kelowna. Highway 3 is maintained in excellent condition, the average driving speeds are substantially less as is the aggressive driving you'll find on the Coq. This route takes longer but is much more pleasant and relaxing. Total driving time would be about 5 - 6 hours (possibly longer depending on your average speed but normally within that time frame).
Thanks everyone for their input. We're going to Kelowna for a concert and returning the next day (Seattle) so the drive is all about getting there & no sightseeing. Taking the advice about 3 being a better road, & that certainly does make more sense than the cok. Actually, we would pull out of Vancouver around 10PM on the 8th and I think Hope (90 minutes) would be a good stop off point for the night. Then we'd do the final 4 hrs on the 9th. We'd leave Kelowna 10PM on the 9th. Would Keremeos be a possible drive/stop over at that time of night or is it too risky and better stay in Kaleden? We do need to be in Seattle by 4PM the next day so we need to drive as much, but as safe, as possible after Kelowna.
Regarding flights, yes we're looking into the West Jet, Air Canada options but it's almost $300 per person, which is more than our budget woud allow.
All of the advice thus far has really been weather dependant. If the Coq is clear with no snow then that would be the quicker way for you. Any snow issues, nervousness about driving during winter conditions then I think Hwy 3 is your better choice.
There are not any hotels/motels in Kaleden so I would rule that out.
If I was leaving Kelowna at 10 pm I would be tempted to, if I was taking Hwy 3, to push onwards to Princeton. There is a Sandman Hotel amongst a few other hotels there. No serious road issues from Kelowna to Princeton. The drive would take you about 1.5 to 2 hours. Otherwise stop in Penticton at the Ramada at the entrance to town (also has a pub) or the Coast.
Kelowna is 45 mins to 1 hour away from Penticton. From Penticton to Seattle it will take you about 6 to 7 hours. From Princeton to Seattle it will take about 4.5 to 5 hours.
Crossing into the US you may encounter delays at the border. Factor some time into this or get an iPhone app for border wait times and follow this after you come out of the mountains near Hope. Probably the Sumas crossing near Abbotsford will be one of your best bets.
Oops, here's the link for the Sandman in Princeton
Another option might be to drive to Vancouver and take the greyhound to and from Kelowna.. you could catch the 12:30 am greyhound on the 9th arrive kelowna 7:10 am then after your concert return to Vancouver on the 11:30 pm bus which will arrive in Vancouver at 5:15 am on the 10th ....means sleeping on the bus but might be easier all round cost would be $98 per person.. you'd then have lots of time to get from Vancouver to Seattle on the 10th.
We have generally found that it takes far less time to cross the border at Osoyoos than at Peach Arch south of Vancouver. If Redtrail's route suggestion does not work for you, then look into crossing at Sumas as BD suggests.
Also, you will find far more choice with respect to accommodation at all budget levels in Penticton or Osoyoos than in Kaleden or Keremous.
If you need accommodation at Princeton, we've stayed here and found it quite satisfactory for a night on the road -