There are several oft-forgotten expenses when you have a canoe or kayak. They include little things like, oh, paddles, life jackets, wet/dry suits, and the biggie - racks. I don't own a large vehicle and transporting boats becomes a bit of a challenge sometimes. (I'll explain in a bit.)
Another thing to remember is that as the driver of the car, you are responsible to see that the load you carry is tied down securely and will not fly off the car. Besides, if you've built the canoe and it comes off the car, some serious depression is in the offing.
First, don't make the assumption that I'm endorsing a particular brand of rack, because I'm not. My brother has had Thule and been very pleased with them. I've had Yakima and been pleased with those. I'm sure if I'd bought Thule brand products, I'd be fine with that. There are plenty of good rack products out there - look for the most flexible (in terms of fitting different vehicles), durable and cost-effectve solution for your pocketbook. I've also seen some really neat home-made racks too - there are lots of options.
After having had a commercial rack since 1991, I have come to realize that in some respects they can be relatively inexpensive over time. I purchased a set of Yakima Q-Towers, 48" wide bars, locks and kayak cradles. They were originally purchased to fit on a Chevrolet Prizm. With a change of two of the Q-Clips and some re-adjustment, they fit a Pontiac Sunfire, and then with 2 other clips, a Volkswagen Passat. Last year, I sold the Passat and bought a Honda Civic. I was desperate for some new Q-Towers as the old ones were showing their age in a bit way from sun damage. The 48" bars finally became obsolete as a set of 58" bars were specified for the Honda. For the amount I used those bars and Q-Towers, they really don't owe me anything.
Here is a picture of the rack on the Honda with a new fairing to try to reduce drag and wind noise - keep in mind that the racks are 32" apart.
Once you have the basic rack, you need to have other goodies to put your boat on the rack. (...or bike, skis, etc. - you get the picture, right?) Personally, for my Wee Lassie canoe, I prefer foam blocks. I actually use them with the rack. As you can see, there are two holes in the blocks - a round-ish one and a wider one. I slide the round part over the bars of the rack and put the gunnels of the canoe in the other slots. When tied down tightly, the blocks behave like springs, keeping the tension on the straps holding the boat down. More importantly, the rack is slick and the blocks make it difficult to get the canoe to move fore and aft at all. It is possible to use just foam blocks on the roof of your car with a good set of tie down ropes - probably the cheapest way to move a canoe, but I've seen people badly dent and scratch the roof of their car by tying the boat down too tightly.
For my fiberglass sea kayak (Yeah, I've got one and no, I didn't build it!) I use the hull cradles. They have built in straps which hold the boat in place. I center the cockpit of the kayak in between the cradles and the straps hold just in front of and behind the cockpit keeping the boat from sliding forward or aft. I've never needed a bow line. It's a slick solution.
This next do-dad is a Hullraiser which allows you to stand a kayak or even a small canoe up on it's side. The cinch-straps came with the Hullraiser, but I use them for carrying my canoe on the rack. When trying to carry more than one boat, this is handy for making rack space. Keep in mind that you have a maximum weight capacity for the rack - mine is 125 pounds.
As I said earlier, the racks on my car are only 32" apart - kinda close together for a 18' canoe. For that problem, I made a home-made accessory out of some 2"x3" lumber, a few bolts a bit of foam and some U-Bolts to hold it onto the rack - works like a charm.
Whatever you choose - keep it safe, simple and reasonable.