Common problem. Until you get to a mid-range device, you're going to be limited in memory and speed. And mid-range devices are a couple of hundred dollars/pounds. Buying used, unless the seller is someone you know and is local, is always iffy. People take upgrades when they're eligible, sell the upgrade for a lot less than retail price, wait a couple of weeks, then report it lost or stolen, They pay the deductible for a new phone and you end up with, at best, a device that will no longer work. (And at worst, possession of stolen property or worse.)
Developing a good relationship with a local independent dealer may sometimes show a diamond among the dust. If he gets a phone for a decent price as a trade for a better one, he may let you have it for a little profit. And it's legitimate.