You have 100% original S4, so it will be fairly better than many refurbished S4 or S5 available in markets. So best try live demos of P8 Lite (available every where) and compare it with S4 side by side and then see.
Huawei or other reputed chinese budget phones are "low maintenance phones", their repairing costs are usually ~1/2 of The Great Infinix and 2013/2014 flagships repairing costs. Plus those are in warranty, so they are safer to buy.
Many experienced shopkeepers frankly told me that they are only 75% times accurate in checking outdated phones. They very frankly admit that many times they get ripped off too while buying used phones with hidden faults. Lol.