Well just a couple thoughts, WPF does increase your damage dealt with a couched lance (such as a great lance), it will also reduce the costs of your upkeep. However I'd suggest going 1h/shield skill to go along with your great lance, so the upkeep issue will be negated by putting WPF into 1h (all melee weapons get a reduced upkeep based on the highest melee WPF). Also, Power Strike makes a couched lance do more damage as well.
I would recommend not using a courser because on some maps where you don't have a lot of open/flat ground, by the time you get up to couching speed you're already flying past the area where you could couch.
Some of the slower horses are actually good for this reason, like armored horses. But the best horse (no surprise) is the arabian because you can stay couched as you are turning at pretty high speeds. If you don't go arabian then I'd recommend a destrier.
These are the two builds I'd recommend for you (depending on if you want a 5 riding horse or 7 riding):
Arabian build (at level 30):
Strength: 15
Agility: 21
Hit points: 50
Skills to attributes: 2
Power Strike: 5
Shield: 5
Athletics: 5
Riding: 7
Weapon Master: 7
One hand WPF: 163
Destrier build (level 30):
Strength: 21
Agility: 15
Hit points: 60
Skills to attributes: 2
Ironflesh: 2
Power Strike: 7
Shield: 5
Athletics: 5
Riding: 5
Weapon Master: 5
One Hand WPF: 146
Now a lot of people would drop some (or most) WM and throw it into IF, however I really like having WPF (especially in 1h). And it will drastically reduce your upkeep costs on melee weapons. Also WPF adds damage to your hits, so having only 5 PS is crucial to have as high of WPF as you can, because 5 PS with a 1h is pretty weak without high WPF. That is something you will have to decide on your own however