You want a +5 Adaptive Seeking Composite Longbow with your choice of Holy (or Unholy or Axiomatic or Anarchic) and possibly Corrosive and/or Shock as well. Unless you are commonly encountering creatures resistant to those (but fire and cold resistance are more common).
Unless you are encountering ghosts often, ghost touch just adds extra cost to a weapon without much return.
There is currently no enchantment that adds sonic damage (except on a crit) I think. COnsidering the crit range of bows, it's not very good.
Do NOT enchant your weapon or arrows with better than a +1.
Instead buy a wand of Greater Magic Weapon at 20th level and add a +5 to EACH for 20 hours at a time.
Spend all your money on other bells-and whistles for your bow.
If your character has 45,000 gp and can find a 20th level caster with the Craft Wand feat who is willing to spend 23 days crafting a wand in the world your GM runs, I would commend your character's ability to build relationships. In three complete adventure paths, I've never seen a party come across a caster any higher than 15th level that wasn't attempting to kill them. =) Then again, your games and the world your GM runs/depicts may be very different than ours. I just wanted to point out that this strategy relies on some assumptions that may not be realistic in every game.
corrosive is a waste of money seeing as the deliquescent gloves give it to you at a reduced cost after a certain point. Seeking is largely marginalized, but I'd keep a spare bow with that on it.
Ghost salted arrows are far cheaper than the enchant in any event, I'd use those instead of ghost touch.
Holy is excellent for most campaigns, I'd recommend this.
Buying your weapon is more about how little you can get away with spending than how much. Greater magic weapon is fine, I'd probably buy several pearls of power for my wizard and ask for Heroism/Greater Magic Weapon/Flame Arrow/Abundant Ammunition/Named Bullet on demand. This ends up being FAR more cost efficient.
If you take clustered shots and ghost salt your arrows you effectively remove the need for some expensive enchantments. There is SO MUCH more expensive equipment you can use outside of your weapon. Getting dependable see invisibility, stat items, defensive stuff, greater hat of disguise for stats/darkvision and pheromone arrow synergy, about 5 great ioun stones, luckstone, wayfinder cheese and bracers of falcons aim.
You might want to consider a higher bonus on it to help overcome DR.my original plan was this, put seeking and designating lesser on my bow that way i can help my party with the designation by giving them a bonus to hit and i think damage and if i missed i got a second chance with seeking. I didnt really plan on brining up my bow or arrows up any higher than +1 cause that is really a waste of money and it really only had a +1 to hit. I know that could mean the difference to hit or not hit but 2000+ gp is alot and it just keeps getting more expensive.
Not all that necessary on a bow when you can purchase or craft cold iron, adamantine, or silver arrows for fairly cheap, although a higher straight enhancement bonus is good for attack and damage.
Atragon, you shouldn't underestimate the value of to-hit.
Adaptive is 1000 gold, flat. It doesn't impede your ability to put other enchantments on the bow and you no longer have to worry about strength fluctuations preventing you from using the bow.
Other than a flat +bonus, I'd consider the Seeking enchantment.
Really would not use any of the other enchantments unless you are in a specific campaign where you know you will be facing one type of foe.
Though, the beauty in this is buying a few human/undead/animal/whatver bane arrows and going to town.
If you have a caster to reliably cast Greater Magic Weapon and other buffs on you then it might be better than increasing the enhancement on the bow, but the bonus to hit and to flat damage is a lot more useful than some people seem to appreciate. As well being able to bypass DR with the enhancement is useful as well. Sure, you could buy non-magical arrows of various metal, or do this or that. But it's icing, not the whole cake.