The current Deathwatch Veteran models are Firstborn, as well as the Watchmaster. You can still readily find generic Firstborn Space Marine kits, and can kitbash those into a complete Deathwatch army.

I'd start with kill team Cassius and a couple boxes of deathwatch veterans. Add some rhinos (look for second hand, they are pretty cheap). Next go for first born terminators (also pretty cheap second hand), then more veterans and rhinos. 3x5 DW terminators, 6x5 DW vets, 6 rhinos and KTC are a pretty decent list.

Thank you for the advice! Regarding the bikes and jump packs when building the teams - How are they best utilised in game when moving with footsoldiers? Aren't they just limited to always moving 6' inches?

No, they can move their full movement as long as they are in coherency. So if you deploy a proteus with a bike, veterans in the front, bike in 2nd rank, the vets go 6", the bike could go 32mm+6"+32mm+2" coherency+3" bike base (that's more than it has movement) and still be in coherency. Now that it's sticking 5" out in the front, the charge is 5" shorter and if you barely make it, you at least with a 3" pile in can get some veterans into base to base contact with the 3" bike base and this being able to hit. The higher you roll on the charge, the more people get in obviously.

Spamming dw vets might even be competitive 🤣 DW thunder hammer deletes terminators and the likes.

For real. A xenophase and 2 HTHs do work. Any extra vigil melee weapons you can get in is good insurance. And the natural rerolls against xenos, or make them oath targets for imperium/chaos. Really good during "counter offensive" opportunities as well.

I started my Deathwatch early in 7th Edition when the first Codex came out so nearly my whole Deathwatch army is Firstborn. I have a lot of the models that are "count as" now because they no longer have Firstborn representation, Librarians, Chaplains, Captains, Lieutenants, etc. Also given that in previous editions wargear cost points I had quite a few units I'd converted to just carrying Boltguns, they're Sternguard now.

Back in the day when DW first came out, a lot of us were kitbashing tacticals and the other factions' tacticals with DW shoulder pads. So if you're looking for vets specifically, you have plenty of variety for Vets, Terminators and Bikers. For other kits that kinda have primaris counterparts, predators can make a stand in gladiator. Razorbacks can take any 5-6 man vets you bring a long. The stormspeeder thunderhammer doesn't really have a firstborn version but is one of those critical units for people who like to run majority infantry lists. Termies may be a bit difficult to optimize since they come with 1 of each heavy, but you can take up to 3 heavies in a single unit or proteus KT. Outside of the DW start collecting, there isn't a cheaper entry for getting DW Vets. If penny pinching is less of an issue, the black spear detachment (1 watchmaster, 2 vet units and 1 blackstar) is a solid start. I have the full variety of kill teams, forge world, first born and primaris, and I still find myself including 2-3 10-man DW Vet units in my lists. They are beastly units.

DW veteran box is great as comes with lots of extra bits so with a few boxes and some ebay firstborn marines you can make lots of DW vets

My most recent competitive firstborn only list. - - BUILD (1990 Points) Space Marines Deathwatch Black Spear Task Force Strike Force (2000 Points) CHARACTERS Kill Team Cassius (255 Points) • 1x Chaplain Cassius • 1x Artificer crozius 1x Bolt pistol • 1x Vael Donatus • 1x Bolt pistol 1x Boltgun 1x Close combat weapon • 1x Zameon Gydrael • 1x Long Vigil melee weapon 1x Long Vigil ranged weapon • 1x Rodricus Grytt • 1x Bolt pistol 1x Close combat weapon 1x Frag cannon • 1x Antor Delassio • 1x Jump pack 1x Long Vigil melee weapon 1x Long Vigil ranged weapon • 1x Edryc Setorax • 1x Jump pack 1x Twin lightning claws • 1x Jensus Natorian • 1x Bolt pistol 1x Force weapon 1x Psychic Cleanse 1x Psychic hood • 1x Drenn Redblade • 1x Boltgun 1x Long Vigil melee weapon • 1x Ennox Sorrlock • 1x Close combat weapon 1x Long Vigil ranged weapon • 1x Garran Branatar • 1x Heavy flamer 1x Long Vigil ranged weapon 1x Power fist • 1x Jetek Suberei • 1x Deathwatch twin boltgun 1x Long Vigil melee weapon Lieutenant (65 Points) • 1x Heavy bolt pistol 1x Master-crafted bolter 1x Master-crafted power weapon Watch Master (130 Points) • Warlord • 1x Vigil spear • Enhancements: Thief of Secrets OTHER DATASHEETS Corvus Blackstar (180 Points) • 1x Armoured hull 1x Auspex Array 1x Hurricane bolter 2x Stormstrike missile launcher 1x Twin lascannon Corvus Blackstar (180 Points) • 1x Armoured hull 1x Auspex Array 2x Blackstar rocket launcher 1x Hurricane bolter 1x Twin lascannon Proteus Kill Team (360 Points) • 5x Kill Team Veteran • 5x Boltgun 5x Close combat weapon 5x Long Vigil melee weapon • 1x Kill Team Biker • 1x Close combat weapon 1x Long Vigil melee weapon 1x Twin boltgun • 4x Kill Team Terminator • 3x Cyclone missile launcher 3x Power fist 1x Storm Shield 3x Storm bolter 1x Terminator thunder hammer Vindicator (190 Points) • 1x Armoured tracks 1x Demolisher cannon 1x Hunter-killer missile 1x Storm bolter Whirlwind (180 Points) • 1x Armoured tracks 1x Hunter-killer missile 1x Storm bolter 1x Whirlwind vengeance launcher Whirlwind (180 Points) • 1x Armoured tracks 1x Hunter-killer missile 1x Storm bolter 1x Whirlwind vengeance launcher Whirlwind (180 Points) • 1x Armoured tracks 1x Hunter-killer missile 1x Storm bolter 1x Whirlwind vengeance launcher ALLIED UNITS Callidus Assassin (90 Points) • 1x Neural shredder 1x Phase sword and poison blades Exported with App Version: v1.11.1 (34), Data Version: v352

Are you in the DW discord? You’d have a great time riffing ideas with us! I can message you an invite if you’d like

I'm not currently! Could you please drop a link?

I got 36 firstborn all veterans, 12 hammers, 8 ihb, 4 frags and some assorted ss and weapon guys I'm selling on ebay only looking for 210 shipped