A suggestion for Autocannon conversion is to pick up an Aegis Defense Line & use the Quad Gun from that. Or, if you're placing a Forge World order, or know someone who is, they have a 5-pack of Autocannon designed for SMs/CSMs that are awesome & only £11.
What I did was pick up the Cadian Heavy Weapon Squad, which has 3 Autocannon in it, then I got the 4th from a friend who had bitz. (You might be able to find the odd Cadian AC at the next Merc Market). They're about the same size as the Quad Gun ACs, but you only get 3 in the Heavy Weapon Squad, but you can probably then sell the remainder of the HW Squad to someone who wants the Lascannons & the guys, and recoup most of your cost. Anyway, the Havocs do end up looking like they're "compensating", but I've got 4 with the Cadian ACs, 3 of the "normal" Havoc ACs that you only get 1 to a box in the Havoc boxed set, and a third, older design. My Havocs are Night Lords, so it ends up looking like they've scrounged the ACs from the battlefield after defeating some Cadians, which is very fluffy for me, at least.
Anyway, that's my suggestion for getting a squad of AC Havocs kitted up. Missile Havocs are really good, too, but if you give them SkyFire they get expensive points-wise. AC Havocs put out a ton of S7 shots that are a big threat to a lot of things on the battlefield, including most armor; they've got a really good chance of glancing pretty much anything that's AV 12 or less to death. One squad of 7 Nurgle AC Havocs is just 162 points, and if you can spare another 7 points in your list you can make them Veterans. That gives you 3 ablative wounds (including the Aspiring Champ), T5 & a pretty good Ld value for when they get shot at (& they will!). That's more shots thnt a pair of Nurgle Oblits can put out for about the same points, and reliably, and you can park them in cover & reach most of the battlefield. I always try to take 2 squads of 7 Nurgle AC Havocs.
Lascannons got cheaper, but really aren't worth it, while Missile Launchers are points effective & versatile, but get pricey when you add SkyFire. Heavy Bolters are good for crowd control, but don't have the reach & Strength of the Autocannons. My AC Havocs have been known to bring down fliers, not consistently, but enough to make me proud of 'em, including a Storm Raven or 2. Yeah, that's a lot of 6s to roll, but it has happened.
Just keep in mind that a good player is going to see them for the threat that they are & concentrate fire on them early, which is why the ablative wounds & T5 helps them to stick around & help out where they're needed.
Anyway, my 2 cents. Sorry if I rambled; it's late, and I have a strong opinion on the subject.