Firstly, 30 Swordsmen isn't necessarily a "big" block, although Swordsmen are generally used for keeping the enemy locked in one place until your heavy-hitters arrive.
Following on that, 15-20 Greatswords is tiny. Remember that you strike last, so you will start taking casualties before you get your own attacks. A 4+ on a Chaos Warrior isn't bad, but Greatswords are also only T3, and WS4. They're not hard to hit, not hard to wound, and they die before they strike. If you run Greatswords at all, run them in block of 30+. Also, Combat detachments work excellently here as they benefit from the Stubborn of their Parent - Swordsmen are a particularly good choice, given their already "tarpit" oriented nature. Otherwise though, I would not run detachments alongside GSwords, because they will count toward Special rather than the minimum Core.
In terms of Detachments, you generally don't want to bother with Handgunners or missile-weapons at all. You cannot move-or-fire Handguns, which means that your infantry will have to stay within 3" of them to gain their benefit and therefore are committing to being locked in one spot. Much better to leave your Handgunners on their own, benefit from the extra drops during deployment, and not have to worry as much about staying in one place. Of course, you can still run them as detachments if you're fielding them in Core anyways - but don't really expect to benefit much from the rules.
It's worth pointing out that a lot of people favor XBowmen over 'Gunners for the extra 6" range, which also helps with accuracy (-1 at 15"+ rather than 12"+) and usefullness. The only real difference is that extra -1 for Saves you gain with Handgunners.
Volleyguns are good, but not much different from Handgunners. The advantage that they provide is that they can pivot on the spot to fire in a 360o arc. They're good for covering your flanks and rear from Ambushers/Fliers. Otherwise, 'gunners all the way. STanks are great, always will be. A touch overpriced when you figure that you could always use Demigryphs instead (always use Demigryphs anyways) but still a solid choice.
As far as your characters, Captains are good just for 'Hold the Line', although I really suggest grabbing yourself a BSB in there somewhere - Empire needs the rerolls, trust me. Also, Priests are a really good addition to any unit that you expect to throw into meaningful combats - Hatred is huge, as are their other abilities. They're cheap and cheerful, run them naked if need be. The Wizard you're running seems lackluster. Damage at range isn't something Empire struggles with, but we do have it hard in combat. Shadows, Light, and Life are always popular options for the discerning general, and Death can have it's uses as well.
I run a gunnery army of my own, based on the College of Engineers. I use a Core of Handgunners (two regiments of 21 with detachments of 10 besides) and a block of 10 Knights for my minimal Core. I back this up with Pistoliers and Outriders, both of which are solid. The Vanguard on the Outriders (or the extra armor) and the 3x Shots at the same 4+ to hit are great, and make it easy for me to reposition my gunfire to where it needs to go, or to hit targets in the back field (enemy artillery etc). Two STanks hold down my Rare, and that's about it really. Characters are a BSB and then a L4 Metal-Mage. It's a hard army to play though - Empire already is a touch weak, but running a gunline like this takes a lot of thinking ahead, careful selection of targets, and a willingness to start "sacking" units after T3 just to buy yourself time and deny the enemy any solid victory points.