I buy the "support company" items as a group, so they are together, as a support company grouping. (It would be a support platoon, if I was playing with a company sized core).
That way I have these key units to deploy first, as determined with my game planning. Not dotted randomly in between the line companies.
It's one of the irritating things about the game that later bought units cannot be "reshuffled" to align with company architecture. So I buy support co stuff first. naturally I'll end up with a few odds and ends at the end of the troop list once I buy a few extras over time, but mainly I tend to stick with the original battalion structure throughout the war. (Maybe if really bored some time, I will make a database reshuffling tool!
)
I
do find some core AAA units worthwhile, as it guards against the times one forgets to buy some (and get whacked by enemy air). They can always fight as great big machine guns if there is no enemy air threat. Or be converted later to more A/T or mortar units (as the Commonwealth did in Burma, historically. ATG and AAA units there were eventually also dual-equipped with 3in mortars as swing-role formations, mainly acting as mortars).
In a German campaign - core AT units can convert to SP-AAA later on when Allied air gets significant.
Having the flak elements have gained experience means that when converted to arty (in a non German core) they then have quicker response times. And any German anti tank converts to flak tend to be noticeably more effective since they usually have a sizeable kill total (and hence the experience gain for that) by the time they do switch roles.
The key arty unit is the FO though, so buy him in the initial buy and never risk him getting near fighting, or in places that the enemy will shell. As he gets experienced, the time to call reduces significantly. That FO experience adds to the experienced core arty troops quick response. So - losing my FO in a campaign is an utter disaster, and experienced mortar sections etc slightly less so.
Things not to have in a core would be helicopters (MBT), any more than a battery of off-map or on-map indirect howitzers per 3 line coys in the core. Engineer tanks and specialist flame tanks that were rare beasties. The German SP-88 on a truck (only 18 ever built, and really were meant to poke holes in the Maginot forts) or the British Churchill SPG which is so devastating in the Desert, but in reality stayed firmly at home, or the German experimental jobs like the SP 128mm. Paratroopers, Commandos, Brandenbergers, SAS and other stuff that only do specialist operations (unless playing a para-commando core force). More than a couple of ammo trucks (one per core arty battery maybe). More than 1 sniper per rifle company HQ, that sort of thing. I also avoid "romper stomper" cores such as converting every tank to a tiger or panther and so I stick to line German AFV such as the P4 and Hetzer, with only a platoon or so of "specials" - otherwise it's a boring walk-over.
Cheers
Andy