Marksman hunters, especially now that they get AP from Int via Careful Aim, should look for "of the Falcon" (Agi/Int) gear while leveling. Marksman has always been a mana-intensive playstyle, as well, so you'll want the mana, anyway. (The "of the Falcon" mail is stupidly expensive on the Bronzebeard Horde auction houses right now.) After that, Monkey (Agi/Sta) is the best option, then Tiger (Agi/Str), Wolf (Agi/Spi), or Eagle (Int/Sta). Straight up Agility or Attack Power doesn't hurt, either. Crit is good for a Marksman. You're not going to find much hit rating gear while leveling, so don't worry about that until you start getting closer to the top-end dungeons. I'd prioritize gear as Agi/AP, Crit, Int, Sta until you get to the point where hit rating and haste start to appear.
Ranged weapons are kind of spotty in old world leveling - they crop up a lot more (and in a lot more variety) once you get to Outland and Northrend. Questing, you'll find one every ten levels or so; if you run instances at level, you'll probably find upgrades more often. There are more bows than guns, but they're both fairly common; there are about half as many crossbows as either bows or guns, though. If you're not particular about which you use, you can carry some of either kind of ammunition and just switch as needed.
Bows: List from low to high
Guns: List from low to high
Crossbows: List from low to high
Your melee weapons are basically stat sticks. Don't worry much about two-handed versus dual-wielding; just go for the best combination of stat boosts you can get out of them.
Gems are mostly a reflection of your stat priorities. You'll of course want to meet the requirements for any meta gems first. If you don't care about your socket bonuses, gem for AP.
NOTE, 9/6/09: Agi has supplanted AP for some reason. I'm not sure on the math, but when AP goes bye-bye in 4.0, it's not going to matter anyway. I believe it has to do with more crits = more Piercing Shots procs, and more bleed damage is more damage.
- Red Gems: AP. If for some reason you can't immediately get an AP gem, an Agi gem is good in a pinch.
- Yellow Gems: Hit if you're not capped, crit if you are.
- Blue Gems: Don't get a purely blue gem. Pick up a green or purple gem instead.
- Orange Gems: AP/crit, AP/hit, depending on your hit rating. Similar to red gems, if you can't for some reason get an AP version, there are Agi versions to substitute until you can.
- Purple Gems: AP/Sta. There are Agi/Sta, AP/mp5, and Agi/mp5 gems if you can't get the AP/Sta one right away.
- Green Gems: Sta/crit, Sta/hit, depending on your hit rating.