Assault Gun Unit upgrade.

[Mobhack]

Towed artillery (or mortars) -> Artillery (including SPA). That one would be best if you want to use units with a high artillery skill and hence faster indirect request response times as Sig or Hummel later.

If you just need AFV (as SP-AT) then you may want to convert something that was originally an ATG. So upgrade your 37mm ATG or flak.

As Germans, I like a few of those really cheap armoured cars with just an MG, a pair supporting each rifle company for anti-infantry work in Poland and France. These are never risked as recce, so the survivors can often have higher skill. They are good for hosing down enemy rifle sections and can zip back to the ammo carriers in the rear echelon fast for a reload if required. They often become stugs or marders, come Russia.

Cheers
Andy