Used (poorly refurbished) Motorola is the best choice (for revenge), if you have a grudge against your father. Most of them come poorly refurbished. Most of them don't even have user replaceable battery and you cannot even swap batteries easily just like you did with your LG G3 when the battery was weak. That's the drawback of used phones with non removable batteries.
Since its your father's first android phone, so he will not be technical enough in troubleshooting, so better get him a brand new flawless and troublfree phone.
Honor 4C is simply one of the best phone in this range. Every phone has its limitations, but its sufficiently good in almost all aspects (regardless of how much phone reviewers on Internet bad mouth it).
Its internal storage is just 8GB, but expandable upto 32GB and apps can be installed directly on MicroSD, so no issues about having less internal storage. However, use of high speed (clsss 10) MicroSD card is recommended for optimum performance.
You can also get Lenovo (parent of Motorola) A5000 or even A6000 for ~14000, but that will not be as good as Honor 4C, except for battery timings in A5000 and 4G in A6000.
Removing original back cover doesn't help much, but aftermarket covers do interfere signal strength slightly.
