Definitely the T-64A. With a bit of thought in the design you could include the gill armour, the rubberised fabric side skirts, the various turret stowage bins and smoke dischargers as separate parts and allow your customers to create the version or update they wish.
If you went with one of the reactive armour variants you would be putting it out of a significant portion of the desired time frame.
I would highly recommend the Zaloga authored Osprey on the T-64, it’s very good.