Okay, first, and you may or may not already know that, but it's standard for people trying to start the game, nearly as traditional as saying "don't start with a
TAG or you will never git gud!".
Infinity has Factions, thing is those Factions have subfactions called Sectorials, those Sectorials lose acces to some troops from the full factions but can deploy bigger quantities of the troops they are left with and sometimes gain access to Mercenaries and/or loaned units, they also can use Fireteams, also called Link Teams, which treat up to five models as a pseudo squad in which they get some bonuses (but have some mobility restrictions and some of them are expensive).
In this case you like Sekbans, which are part of the Qapu Kalqi (
QK), the Sultan's naval forces, and the Azra'il, which are also part of the Qapu Kalqi, the Hassassin troops on the other hand are part of the Hassassin Bahram Sectorial (
HB for short), along with the Asawira Regiment, so if you want to have both things Vanilla is the way to go (althought you could collect both Sectorials, but let's say you want Vanilla).
You can look into the
Vanilla Haqqislam Starter Pack, which includes
"Solid Snake" Ghulam, "Professional Looking Guy" Ghulam and "Starter Set Girl" Ghulam, good line troops which, thanks to the new Edition (you alreeady know about the new edition, right? of not ask away) are quite good at guarding corners (their Rifle+Light Shotguns are quite versatile, and Shotguns have a great bonus at close range) and at healing (they heal on 17s), and provide orders for a good price (not the best but at least they went to school instead of playing shooters, working in the air conducts or watching Martial Arts movies), a
Naffatun with Rifle+Light Flamethrower and Grenades, Flamethrowers are rather good but not as much as in the last edition, still, fire ammo is still powerful and Grenades are a good, albeit situational tool, you also have a
Janissaire with
AP Rifle+Light Shotgun, he is a solid Vanilla Heavy Infantry, althought pretty vanilla, can also be a Doctor (a combat Doc as some calls them, a guy who will be in the frontline anyway so you won't feel like wasting orders for trying to save the life of some guy who fell upfield) and can get some protection against hackers for a price, lastly you have the
Hunzakut, he is a skirmisher with rather good options like a Grenade Launcher and Deployable Repeaters (for hackers), a Sniper Rifle profile and a Forward Observer option with Rifle+Light Shotgun, wihch is rather good for a Skirmisher that can start deployed on any point of your half of the table.
This should give you a core to build around and to play, on the other hand you may want something different, or simply a group of miniatures you can play from time to time so let's look at waht you specifically asked
1-
On the Sekban: They have rather nice miniatures, in last edition they had some problems because of the large numbre of great troops in their point range and the shame of being Medium Infantry, on one side the shame is not only not gone but greater because only
MI and a few of the heavier
HIs are slower than usual, on the good side the changes on point costs, some weapons and some general rules have given them a new life, they aren't the best troop of the game but they seem solid to me, they carry a pretty varied kit, namely 360 visors, which interact nicely with the Suppression Fire mechanics, and their choice of pistols is situational, as is their integrated Shrapnel Dispersers (Chain Colts).
2-
On the Azra'il: They are going to be resculpted (althought there are talks about it being little more than a re-escalation) due to some profile changes done in the new edition, they went up in Armor and got a bit of an upgrade in the firepower section at the cost of losing one of their three profiles, but look good to me.
3-
On the Hassassins: In general they provide some good tools for Vanilla, elite
LIs (Muyibs and the new Govads), powerful snipers (Lasiqs with Viral Rifles/Sniper Rifles), the dreaded Impersonators (Fidays and Hussein Al-Djabel) and great Hackers and Hacker support (Barids).
4-
On the Asawira: Elite
HI, they have above average Close Combat skills (just don't try to kill three
CC specialists at the same time, their fluff is a bit over exagerated, aim for eaier preys) and the best shooting skills of the Faction, on par with the Maghariba Guard, they took a bit of a hit in the changes to their Regeneration rule but also went down in price, they are no longer super great but are a good elite
HI, and they have awesome miniatures.
now, on my suggestions, don't take them too seriously because
1- metas are different
2- I'm Spanish, metas are really different
3- Althought I have been playing Infnity for a bit Haqqislam is a faction I don't have great knoweledge of so most of what I say is armchair general talk and extrapolations from different units form other factions.
Buy what you like, all units are good.
Now, this is not an absolute, rule look at this threadon basic listbuilding concepts:
http://infinitytheforums.com/forum/index.php?/topic/12350-list-building-in-infinity-or-why-you-see-banners-proclaiming-it%E2%80%99s-not-your-list/
and maybe play a bit with one of the nearly fully updated army builders
http://anyplace.it/ia3/ia.html
Oh, and wait for the rest of the Infinity Dakka community to post on this topic,
iIrc loki and some other people play Haqqislam so they may be able to give you more advice.