Some of these formations are available in the Space Marine codex, whilst others can be found in the Angels of Death supplement, which collates and updates formations from various other earlier supplements and even a previously limited edition datasheet - the infamous Skyhammer Annihilation Force.
Remember that formations may be taken as stand-alone forces in their own right, which could be your whole army or serve as a bolt-on to another formation or army. There are a couple of formations in the codex that start at around the £50 mark to start with (Reclusiam Command Squad, 10th Company and even the Librarius Conclave). However, whilst these are potentially battleforged armies in their own right, they are best taken as part of a Gladius Strike Force (P112 in the codex).
This special type of detachment is commonly-known as a Decurion-style detachment, as it was first introduced in the Necron codex under that name. Unusually, these are detachments made up of other formations of units, rather than individual units. Typically, there is a minimum requirement of a core formation alongside one or more auxiliary formations. Command formations are also available.
For Space Marines the Battle Demi-Company is the basic core formation around which the Gladius is built. The minimum requirement for this is a Captain or Chaplain, 3 Tactical Squads, an Assault element (Assault Squad, a choice of bikes, Landspeeders or Assault Centurions) and a Devastator element (a traditional Devastator Squad or the newer Devastator Centurions), as well as options for Dreadnought and a Command Squad. Whilst this gives a decent selection of options already, the minimum price of this is around the £100 mark. 3x£25 Tactical Squad boxes will give you just about enough marines to field minimum-sized squads for all of these slots, if you get creative and make a Captain and your assault and devastator marines using the respectable range of weapon options available. Purists would argue that Marines should always be fielded in squads of 10, but units of 5 are perfectly legal. Your next purchase should then be the versatile Devastator (£28) kit, followed by either a unit of Bikes, a Landspeeder or an Assault Squad (£15-£20). The Devastator squad is particularly useful, as it contains lots of heavy weapon choices to spread around your tactical squads. (You might want to keep an eye out for the discontinued Demi-Company box, with all the options for £135)
Your next decision is which auxiliary formation to go for as the minimum for a Gladius. In order of basic cost, these are roughly:
10th Company (£45)
1st Company (£75)
Suppression Force (£90)
Armoured Task Force (£105 if you convert a Techmarine from a Rhino gunner)
Stormwing (£116)
Anti-Air Defence Force (£120 or cheaper if you convert a Rhino with a spare weapon)
Centurion Siegebreaker Cohort (£133)
Land raider Spearhead (£135)
Strike Force Ultra (£253)
Each of this choices will change the flavour of your army and should give you some fun games with a good variety of units.
The first benefit of a Demi-Company is that each unit in it gains Objective Secured, which is very useful in objective-based games. The doctrines are also handy in the right situation, but you do need to remember to call them at the correct time. Where the Gladius becomes really powerful and a headache for your opponent is with two Demi-Companies, when you can then take basic Dedicated Transports at no extra points-cost, all of which also have Objective Secured. The cost in hard currency of this is potentially even scarier. First off, you clearly need to spend double your initial outlay to get two Demi-Companies. The transports then cost £22.50 (Rhino or Drop Pod) or £25 (Razorback) each and you could potentially get up to 14 of these.
I had intended to go into some of the formations in Angels of Death here, but I think that is best left for another post.
