The older models are probably not updated to Google Play as the underlying TV hardware will not run Android.

Simple as that I'm afraid.  Unless your TV has a Qualcomm / Mediatek / Intel or similar Arm or x86 processor and the requisite amount of RAM to run Android then it will never be able to be ported over.

You would also need a minimum amount of Storage space to install the ROM ( My Nexus 6 Custom Android 6 Marshmallow Rom is only about 350ish mb download size, but some versions of Android Installs can weigh in at Nearly 2GB like my old Huawei Ascend Mate 2 ), you need storage space for the Rom, storage space to actually install it after you have downloaded it, and then space again to actually install apps and cache data.

Your TV would need to have a minimum of 8GB storage space for an Android install to be viable even if you did have a compatible processor etc.