That's quite a task you've set yourself! I've long toyed with a similar notion but the amount of work involved & other commitments has so far put me off (that & hope that the JTFM After 45 range will appear at some point).
As to which tanks could make good doners;
Panther F- fairly easy, you've got the Heer 46 turret you mentioned plus a variety of Panther kits to put it on.
E-50, well a Tiger II hull is a much closer analogue than a Panther (it starts to get a little expensive from this point) - if you wanted to get more accurate running gear it'd be easier with a plastic kit like the Tamiya, as Dremel-ing out roadwheels from a resin kit, while do-able, would be challenging & not necessary if you just want 'essence of E-50'.
E-75, this would be the easiest, as running gear aside, a Tiger II kit would do, just use some plastic/brass rod/tube to represent the 105mm gun.
E-100, tricky, all the problems of the E-50 & bigger with it, I suppose a 1/48 Tiger II kit might be closer to the right size, stick the Blitzkreig Maus turret on it (& if you've a mind to, scratch build the skirt armour) - personally I just wouldn't, buy the Blitzkrieg Maus (an excellent kit) instead untill someone tackles the beast.
Heer 46 does a Lowe (in 1/48) which looks pretty good.
Sorry to come across as a bit of a downer
but at least the Panther F & E-50/75 would be easily do-able with available models, as both the latter could be based on a Tiger II hull, good luck with the project!