Standard Tech-Priests are fine; I recommend the 32mm-based ones (Enginseer, Datasmith) as Tech-Priests on foot and the plastic Dominus on the 50mm base as a model on an Abeyant and/or as an Archmagos to justify him being so much bigger.
You don't
need FW Secutarii parts; Peltasts are allowed radium carbines so just building a Vanguard squad is technically a legal 30k Peltast squad (do keep in mind that arc rifles are allowed but the plasma caliver/transuranic arquebus aren't). A purist might argue that the backpacks or the helmets are wrong, but reasonable people would be fine with it.
The other
HQ option you might consider is the Axiarch; you'd need another Skitarii model to convert into one (so two boxes) and you'd need a cortex controller from another source for your Thanatar but he gives the Secutarii some free buffs.
Your other option for adding a few battle-automata to a Legion army is to just take a Praevian; he gives the bots some bonuses depending on the Legion, and some of them are really good (ex. Thousand Sons Castellax with 4+ Invulnerable saves). The best are the Raven Guard and Alpha Legion, though, since Independent Characters confer Infiltrate to their attached squads they get to Infiltrate the Battle-Automata and
then add the Praevian bonus.
Unfortunately Legion cortex-controllers/Praevians only allow Castellax, Vorax, and Domitars, so no Thanatar if you take that route.
If your major objective here is that you really want a Thanatar and are searching for the cheapest ($) way to add it to an army I'd suggest going back to your original plan and get a box of Skitarii, a Tech-Priest Enginseer or Tech-Priest Dominus, and the Thanatar. Run the Tech-Priest as a Magos Dominus (Rite of Eternity out of Cybertheurgy is a more reliable way to repair the Thanatar than Battlesmith, and Patris Cybernetica lets him hide behind the Thanatar so you don't need to hold the Secutarii back as a bodyguard), take Enhanced Targeters on the Thanatar (+1
BS and an extra -1 to the target's cover), and you're looking at 460pts for the whole detachment.
Automatically Appended Next Post: 
SirDonlad wrote:The techpreist can serve as a low level
hq for sure as a Magos Dominus for Thanatar herding duties.
To use secutarii you have to have a Taghmata or Ordo Reductor force and you have to include 'a titan of any class' - otherwise you can't use them to fill compulsory troop slots.
My suggestion is to re-prpose an imperial guard squad as Tech-Thrall Adsecularis Covenant for the compulsory troop slot or splash out on a Thallax Cohort.
I keep forgetting the no-compulsory-Troops restriction; I usually use Secutarii in my Ordo Reductor army where I've already got a couple of units of Thallax.
Other cheap sources of Troops: Converting Adsecularii is certainly an option; they tend to be beefier/more muscular/less dressed than Guardsmen, so greenstuffing mechanical bits onto Catachans might work better than Cadians. A Macrotechnica Magos would let you use Tech-Priests in Troops; the plastic blisters aren't cheap but you only need one plus something to use as servo-automata. If you bought a Kastelan box you could probably get away with using the Datasmith as a Magos Dominus and the Kastelan as Castellax, then define the detachment as Legio Cybernetica so the Castellax can be compulsory Troops. Thallax aren't cheap ($50 for a team of three last I looked) but honestly they're not that much more expensive than the other options on this list, so...
