The books Scars and The Path of Heaven, both about the White Scars during the Horus Heresy, have inspired me to build a small fleet. Probably not a full fleet, since I have 1500 points of Blood Angels in progress. Good books, by the way. Not enough space combat that I'd say you should read them just because you play BFG, but there is some. It's also the first time I've identified with a legion (or M41 chapter) of Space Marines - aloof, contemplative, disorganized, none of that "not one step back" attitude.
Anyway, a lot of the HH books mention Gloriana-class battleships - every legion seems to have had one as their flagship. But there isn't much information yet, and even less consistency. Forge World's third HH book has a couple of profiles of Iron Warriors and Imperial Fists ships at the Battle of Phall, including an IW Gloriana-class battleship that look roughly like a battle barge but bigger (they show it next to a Cobra for scale).
It's a more official source than the Maccrage's Honour visual novel, which portrays the Ultramarines' Gloriana-class flagship as an Emperor-class ship. The Visions of Heresy books have artwork that looks old and outdated, but it shows a number of legion flagships (including Horus's Vengeful Spirit) in the Imperial Navy battleship style too. The White Scars novels don't go into detail, but they do mention identifying their ships by sight by their white prows, which also suggests the IN aesthetic.
That FW book also shows the Contrador, a "Legate-class" Iron Warriors battle barge that looks like a compact battle barge, and the Tribune, a "Victory-class" Imperial Fists battle barge that looks like a Vengeance grand cruiser but with an armored prow, torpedo tubes, and a "planetary assault petard" - a big dorsally-mounted gun-looking thing.
The White Scars novels say that all of their ships have been modified for speed (many of the primarchs emphasized one aspect of design), at the expense of lance range and troop/dropship capacity. The books also mention lances a lot more than other weapon types. I'm tempted to build an Apocalypse, but use Desolator rules as a Venerable Battle Barge. That way I get my fast battleship with lances that don't shoot as much as they should (4 each side instead of 6) because all the power's being diverted to the engines. The Desolator's torpedoes also make more sense than a nova cannon in M31. The downside is that using one ship as another is confusing. It irks me, but I can't see the Desolator model looking the part - it just exudes Chaos, both in the details and in the overall shape.
Escorts will be Novas, since the main character is in a frigate with a single prow-mounted lance. I might turn to Shapeways and get some Grey-Knight looking strike cruisers and remove the symbols and bridge, and stick old Chaos cruiser bridges on them.
Sorry about the wall of text.