I would say collect 28mm by all means but vehicle wise do Tamiya 1/48 or if you can find them Corgi diecasts (1/50 scale). I bought a 1/56 scale jeep and I was totally disappointed. 28mm 'heroic' figures just look totally out of scale with 1/56 jeep. In reality you can have 4 very comfortably in a jeep-even 8 is doable -compare an Artizan or plastic Bolt Action figure by a 1/56 and is just not happening. Frog do a plaggy jeep in 1/48 for around a fiver, Tamiya tanks range from £15 to £28.
On the otherhand you could do 20mm and then its Pegasus Waffen SS, AB Figures for Brits, Millicast, Italieri for vehicles etc etc etc....