1. How do you bring buildings? Do they have their own "detachments" or are they limited to CADs?
2. What are some good buildings to bring for fluffy Fists?
3. Are they even really worth bothering with? From me brief look at them in battle scribe they seem fairly overpriced for shooting platforms.
4. What kind of point values do they lend themselves to well/poorly?
If you want to go 'fortification heavy' then I'd go with the Imperial Strongpoint - it's 1-3 bastions, 1-5 aegis lines, 0-1 skyshields and 0-1 honoured Imperium.
They look best with Fists - the Wall of Martyrs stuff is a bit too baroque and too many bits have dead cadians sculpted into them. Besides which, the cover of stronghold assault has an Imperial Fist fighting from behind an aegis line...
In a battleforged army (obviously unbound you bring whatever the feth you like), you have 1 fortification slot per Combined Arms Detachment, however there are several 'fortification networks' which give you multiple fortifications in a slot.
I'd suggest putting them in at 1500-2000 points.
Are they any good? Well, purely as a firing platform.....sort of. A bastion is cheap and has an automated fire heavy bolter in every direction. It also has 3 firing points in one direction, is as tough as a land raider, and is immune to graviton weapons. Plus, it gives you a cheap 'slot' for an Icarus lascannon. A single Icarus is no big deal, but 2-3 of them is enough to make flyers seriously reconsider their life choices, especially if the accompanying aegis lines pack quad-guns to boot.
One of the most important things to understand is that fortifications can be an offensive weapon as well as a defensive one. The most important upgrade in this regard is the escape hatch. Buy one for a bastion, and deploy the bastion at the front edge of your deployment zone. When you deploy it, you need to place the escape hatch within 12". This is another access point for the bastion, and the escape hatch marker is a couple of inches across
Imagine deploying a unit of assault centurion warsuits (a fairly so-so unit but very appropriate for the Fists) in the bastion. Turn 1, they can disembark within 6" of the escape hatch - which means about 20" up the board (12"+2"+6") - that's enough to put their hurricane bolters within rapid fire range on turn 1 - and potentially even get flamer shots turn 1 if your opponent has advanced up the board.
Certainly, a bastion with an escape hatch gives you a way to get footslogging units well past the midpoint of the board on turn 1, and thanks to the building rules they can still attempt a charge if a target presents itself. All for 100 points, and the bastion itself remains safe(ish) in your deployment zone, where a second unit can embark ready to follow the first wave.
Two 'assault tunnel' bastions can shuttle a big force up to the enemy's lines very fast, and provide a nice save firing platform on the battlements for some heavy weapons whilst they do so.
Also remember that the building upgrades can be very useful. Ammunition stores are a viable alternative to quad-guns, giving units behind the lines improved accuracy (probably irrelevant to Imperial Fists though with Bolter Drill doing much the same) but Tank Traps are worth their weight in gold.
A line of cheap obstacles that are impassable to non-skimmer vehicles, you say?
An alternative (or take both!) is the Void Shield Network; it's a load of void shields and promethium lines. If you've got a defensive line of infantry behind aegis lines, a single promethium line turns the handful of flamers into terrifying squad-burners, whilst several void shields can carpet your entire fortification zone in shielding, seriously cutting the teeth of destroyer weapons and graviton cannons.