You are really hurting for anti-armor with what you have now. So if you want that Sicaran Venator, you will need at least two Fast Attack choices. Let's talk about your questions then address the flyers.
Are Seeker Squads any good?
They wold be if you could take jump packs for them. But you can't. You are paying a premium price for Special Issue Ammunition that is nowhere near as good as what Sternguard in
40K get. Look at the ammo. Tempest Bolt Shells: R18" S3 AP6 Rpaid Fire, Blast 3". Don't have to roll to hit (yes!), they they scatter (boo!). Wound on a 5+, and target get their armor save. If you fight a lot of Solar Auxilia, this might be the shot to use. Otherwise... gak. Kraken Bolt Shells: R30" S4 AP4 Rapid Fire- so... a bolter round with a little extra range and one less
AP, vs. Legions with 3+ saves. Again... not so good. Scorpius Bolt Shells: R24" S5
AP 2 Heavy 1 shred. The only ammo that is worth it, but that heavy 1 really puts a kibosh on the whole thing. If you are shooting at your preferred enemy, it might be a thing. If you could make snap shots better, it could be a thing. But ya know what? A Tactical Support Squad with plasma would be just as good, get more shots, and be more flexible. Precision shots are useless in 30K. The only guy that may have different wargear from the rest of the squad is a Sergeant, and he'll most likely have a 2+, and Look Out Sir. These guys need a transport to get anywhere, and if they are in a transport, that means only two shots from the squad in a Rhino, or they are sitting off the table in a Drop Pod waiting to come in. Nope. For your intended purpose and the army you want to run, Seeker Squads are not for you.
Javelins These could work- but they are a bit fragile. I'd probably keep them stock, as the Cyclone
ML has a great profile and lots of flexibility. If I needed more anti-tank, I'd upgrade it to the
TLLC. I would not give it the heavy flamer or
MM- both require you to be too close to the enemy, and the abundance of Armor Ceramite makes
MM's not a good choice against a lot of the vehicles that can take it (and they always take
AC- it's almost a no brainer). Strafing run is awesome (
BS 5! Woot!). Being able to stay in reserve and then out flank could be very handy against some of the bigger threats out there. This is one of those units where I'd probably give them
HK missiles. For your intended application, two of these to fill up the required
FA slots would work, and give you some much needed anti tank firepower.
Dreadclaws Here is where
FW failed the Sons of Horus. The SoH Dedicated Transport Anvillus Drop Pod costs only 100 points- a 15 point reduction and it's a Dedicated Transport that doesn't use up a Force Org slot. Read the entry carefully, then read the
FA version of the Anvillus and notice how the SoH version does not have Inertial Guidance System. Ever lost a Justaerin Squad in an Anvillus due to Deep Strike mishap? I have, and it wasn't fun. Pay the extra 15 points for the
FA version. Now then! If you take the Anvillus, what are you putting in it? Because it's a shame to waste all those points on an empty transport that has the assault vehicle rule and can burn units with Heat Blast. Justaerin? They can Deep Strike in the Black Reaving. Horus and the Commmand Squad and an Apothecary? Now that sounds like a fantastic idea, as long as your opponent doesn't have any Anti Air- Crash and Burn could be brutal. So what to throw in the other pod? I'd say Reavers. You'll want to keep your Legion Tactical Squads on the table for as long as possible for objectives. Anvillus Drop Pods are kinda expensive though- both in points and $$. However, as you have no other transport option for Horus, your
CS, or
HQ's like a Spartan Land Raider, I'd say the Anvillus is the way to go for you.
Tarantulas Not a bad choice if all you want to do is fill out your
FA slots. Don't expect them to do much, as they have crappy
BS and bad, bad auto target rules. I would say before you go the Tarantula route, get some Land Speeders instead. You can use the MK III or whatever it is from
40K (they were present during the Horus Heresy), and save some money. This would be the best option
IMHO if all you wanted to do was fill out your
FA slots.
Bikes I'm glad you didn't mention Space Marines on bikes as they are the most ridiculous unit ever in 30K or
40K. My 10" genetically created super soldier with power armor is going to ride into combat on a motorcycle. Ugh. But if you can get past that mental image and how absurd it is, then a couple Attack Bikes to fill out your
FA slots or Outriders could be worth it. And again, you could use the
40K models to save some cash.
What's left? Flyers. You said no flyers, but for this particular application, flyers would be the best way to go. A Xiphon and a Storm Eagle... WOW. Horus, with his Justaerins on the Storm Eagle, ranks right up there with a Spartan Land Raider for an effective Primarch delivery system. Or a pair of Lightning Fighters. The great thing about flyers in 30K- if your meta doesn't prepare for them, they are devastating. If your meta uses a lot of AAA- then not so much.
Reavers I'm not a big fan of Reavers, simply because they cost more than a Veteran Squad and can't do nearly the same functions as a Vet Squad. In the Black Reaving, you want to take
FA choices, and a solid choice here is a Reaver Squad, but they can count as Troops Choices, and if you take Maloghurst, they are Troops. So...
WTF Forge World?
Reavers are OK if you give them chain axes and jump packs, then either
DS them or Outflank with them, but still! They cost more points then a Vet Squad, have less options then a Vet Squad (but at least Chain Axes!!! Rule of Cool in full effect), and can't pick a skill like a Vet squad. I want to use them, but every time they disappoint, and I wish I would have used a Vet Squad instead. Making them into Troops makes them a bit more palatable. It's just a bit irksome to me that the signature squad for the SoH is pretty lame. I always take at least five of them though. Rule of Cool- how could you not take such cool looking models with Chain Axes called "Reavers"?
Recommendations For your intended army list, I'd say two Anvillus Pattern Drop Pods as Fast Attack Choices with a Reaver Squad in one of them, and your
HQ with Command Squad and Apothecary in the other would be good. Have your Master of Signal join a Tactical Squad with an Apothecary, have them sit in cover, and help with all the Deepstrike stuff.
If you are just filling in slots, take two Land Speeders. Stupid cheap both $$ and points, and they might actually do something on the table. Heck, I might even give them an
HK. Wouldn't spend any more points than I had to though.
Most effective thing you could do: Either a Lightning or Xiphon, and a Storm Eagle. Most expensive points wise and $$.
Good luck, and Lupercal!
