I think I'd have to go with the flak turret as an add-on, the huge points cost of heavies in pretty much all games at 28mm (with perhaps the Tank War BA variant being the exception) precludes putting more than one on the table.
While SoTR has rule for the smaller E's (10 & 25), & it being easy to substitute them for Hetzers & Jagdpanzer IVs (pretty close real world analogues), the E-50 is essentially a Tiger II (equivalent gun, armour & performance) & can so be substituted for that. The E-75 is a tougher call - it's a Tiger II with nobs on & I don't think there's a straight crossover in any ruleset (bar Victory Decision?). You'd have to find a way of representing more armour than a Tiger II &, perhaps more importantly, it's 105mm gun (even if it were a straight replacement with the existing 105mm AA weapon it'd have more poke than the 88mm KwK43, without attempting the impossible task of trying to cram the KwK44 128mm in, along with the huge two-part ammo & the extra loader crewman necessary). If it were the proposed AT version, it'd probably have been significantly better - much like the comparison between the Tiger I's FlaK 36 & the Tiger II's PaK 43 based weapons.
That said, want one? Silly question, of course I do! I was planning to use the spare tracks from the E-50 full kit to attempt a scratchbuild of the (fictional APC) Buffel, but were you to have a stab at that...