Depends on what you mean by competetive. If you wish to play 8th edition you almost can't do without at least one huge block of infantry and at least one magic user.
Further, it depends on the amount of knights you can field. An army composed entirely of the units you described is very vulnerable to an army fielding lots of heavy cavalry. As soon as your gunline gets into combat, they'll be rolled up quite easily. You should try to fit in at least a capable unit of swordsmen or halberdiers to protect the missile troops from cavalry or flyers and units such as miners, tunneling teams and the like. Due to gunlines being fielded predominantly as linear formations (and being not as heavily armed and armoured as combat-oriented troops), they break easier and are quickly overrun in close quarters. Unless your cavalry is capable of occupying each enemy unit capable of quick advancement, a block of troops is very versatile in defending your artillery and gunners.
Should spears become a viable option again in 9th edition, you could possibly convert a unit of gunners to wielding muskets with plug-bayonets, to keep your army themed.
The same still holds for older editions (which I play) though the units you use don't have to be as massive as in 8th.
A quick run through the rest of your units:
Handgunners are devastating against light and medium infantry/cavalry, but struggle against heavy infantry/cavalry. Heavy Cavalry is especially dangerous because they can close with your gunners very quickly.
Pistoliers are my favourite unit, though I'm unsure how they work in 8th edition. They're fast and can easily bait and flank units, denying them marching capability thus slowing them down, which is very good for a gunline army. Additionally, they pack quite some firepower, making them deadly harrassers. Beware of enemy missile fire, they're still light cavalry you know!
Outriders are very useful as well. They're basicly a mobile firing platform, laying down as much fire as an entire gunline! They're not as broad as a gunline, thus are very manoeuvrable and don't get in the way as much. Try to move them as little as possible, because they can't shoot if you move them. Get them were you want them to be and lay down fire for as long as possible. They're quite expensive however, and take up valueable special choices/points.
Cannons are okay. They're the only thing capable of reliably wiping out heavy cavalry and monsters, and can be used to snipe enemy characters if you're lucky. Use pistoleers to trick enemy knights into showing their flank to your cannons and watch as a well-placed cannonball wipes out the entire unit in one shot. Like most artillery however, they're a two-edged sword. They often don't kill as much as you'd like, and have a tendency to blow up at unfortunate moments.
Rocket Batteries are a bit like overgrown mortars. They scatter wildly however, meaning they almost never hit a thing unless you're very lucky, or if you fire into a very high concentration of units. If they hit, they slaughter light and medium infantry/cavalry very well.
Knightly orders are very basic heavy cavalry. They'll do the job, but will fail against their superior counterparts, such as chaos knights, bloodknights or any brettonian knights, unless they outnumber them and are bolstered by a powerful character. Heavy cavalry can be used to quickly destroy enemy missile troops, or to smash through weak units in the enemy line and then charge larger units in the rear. Artillery is their greatest fear, but in the mean time there's no better unit to take out artillery quick and easy.
Using only captains will certainly limit your magical capabilities, both offensively and defensively, which in some games is a major handicap. A warrior priest or battle mage might bolster your magical defenses, and perhaps give you some offensive capability as well. Master engineers can be used to make cannons somewhat more reliable. Captains might bolster the fighting ability of a unit gunners, perhaps enough to withstand attack from some smaller medium or light infantry wittled down by fire, but absolutely shine in more combat-oriented units.
There, my vision on those units. I'm by no means an expert, and really not up to date on the latest 8th edition stuff, so take my advice for what it is.
I myself would include a unit of plug-bayonet gunners as count-as spearmen or halberdiers, just to add a little protection to your gunline and to finish of any unit surviving the storm of lead.
Gunline armies are not the most exciting to play, as movement is what makes (pre-8th edition) Warhammer great. The more cavalry and combat-infantry you'll include, the more exciting it'll get. None the less, a gunline army can be very thematic.
I hope this is of use to you.
Last edited by Tha Tall One; 10-21-14 at 09:41 PM.