Assuming you are starting in September 1939, and are going for an understrength battalion core (down one company), two good references for what assets are available would be:
infantry or
motorized infantry.
Rather than use the 5cm GrW units, I normally use the (sw) infantry unit that has primary weapon rifle and 5cm GrW in slots 2 through 4. While pretty useless in indirect fire, the 50's aren't too bad in direct fire as support. From what I've read it was fairly common for a battalion commander to break up the machinegun company (which had the 8cm mortar battery) into 'per company' attachments. So in addition to the 5cm, you could quite historically have an 8cm section as well as an extra HMG section.
Early on at least, the 7.5cm infantry guns (as well as the ATGs) tended to be regiment assets but it doesn't seem totally out of the question to split them among the battalions giving you a pair of 7.5cm infantry guns and three ATGs to reinforce your understrength battalion. The 7.5cm infantry gun is actually a pretty handy piece early on, used together with your 8cm mortars it gives you a better organic indirect fire capability, but also is decent as a long range direct fire weapon against infantry and, in a pinch, a short range AT capability against slow moving lightly armored vehicles.
You will probably also want to add a trio of 2cm FlaK guns, they were detached from Luftwaffe assets to Wehrmacht divisions and generally you could count on at least one per battalion. These are also good in their secondary role as long range ground support and light AT.
Another addition, while not quite historical (but not too far out either) is to add a 10.5cm light artillery battery to your core. These were division assets and were assigned on as needed basis but buying one with support points every turn gets tiresome and limits your support choices.
Don't forget to add some recon elements and a FO, you could attach some division recon assets such as a motorcycle squad or two and a troop of armored cars. If your infantry is part of a
panzer division or
motorized light division, you could reasonably add a section to a platoon of light tanks. The tanks would almost certainly not be permanent attachments but once again like the light artillery battery, gives you more flexibility with support points.
Hope this helps,
Brian