The Luchs is not the first unit you can produce as OKW. You can plant mines infront of your base, get AT guns into position etc or even tech for a AEC which comes at the same moment.
Neither the M3. At most, it's your 4th (T1 - 2 Conscripts - M3) or 5th unit (2 Cons - CE - T1 - M3). Flamer arrives at 60muni and guess what, OKW now has equal muni income. This means you can put 1 or 2 mine at least till your first HQ is deployed. Depends on whether you want to rely or not on early schrecks or rak.
Small arm fires can deal fine against an M3 if you don't spread out too much your army. Volks do more damage, SP and Kubel are cheaper and you have more initial mp as well.
Tip: you can still see the xp kickers on FoW. I'm not sure if there's a difference between T2 and T1 but early on when there's not much to do, i sometimes roam around an SU base to know what i might be facing.
Ontopic: Flak HQ should had lower AA capabilities. Just give them normal bunkers.