The best bang for your buck would be to buy the Hammerhead / Skyray kit. You could then buy the devilfish nose turret off of eBay for a couple of dollars and be able to have all three tank options.
Alternatively, you could purchase just the SMS turrets off eBay (sans the pods), and stick SMS pods in the turrets themselves if you have a few laying about. You may have to clean it up a bit, but it will work and look right.
As far as tactics go, the SMS on a devilfish is not usually taken. It has the capability of giving the devilfish more of a punch, but the vehicle is not really brought for it's shooting capabilities.
Sky IS Falling, T'au WILL Suck, Sell Me Your Models