A nice addition to any collection, and the 1a4 was my first tank. I've got both, and they are fully operational after considerable reconditioning.

A fully assembled and painted Tamiya Gepard and Leopard 1 a4 should be worth about 300-400 dollars each (that's full retail)...and that's if they are in running condition (look good, run good and no missing pieces or manuals). I assume they are the double clutch versions (original issued) and not a static version hack. It sounds like they've been modified with modern electronics, so they wouldn't be collector's items. Collectors want them unassembled, or absolutely stock. In a modified state with modern electronics...they have value as a runner only. Still, they are both very unique tanks that are a nice addition to anyone's collection.

Check the Gepard to make sure the cannons aren't broken (especially look at the bird cage at the tip of each cannon) and that the radar dish motor works. On the Leopard, check the barrel. The original Leos had an aluminum barrel unlike the recent static kit's plastic split barrel halves. Make sure it's got metal there. Check the commander and loader hatches on the Leo. The cupola rings (the rail around the hatches) tend to fall apart and get lost. On both tanks check the road wheel tires, as they need replaced every 20 years or so...and if original, they'll be hard and cracked. Parts are notoriously difficult to get for them, so find the replacement value of the missing and damaged parts, and discount the sale price accordingly.

If you want full option versions, I'd recommend starting with the static kits and modding them up to full option. It would be cheaper.