When buying a used unit, the condition of the particular unit is far more important than the initial build quality. A 5 year old well built unit in poor condition is unquestionably a worse buy than a cheaply built 5 year old unit in pristine condition. Water intrusion is the RV's worst enemy and some water damage is very expensive to repair to the point costing more than purchasing a new unit. You mentioned the slide having a 4" gap. Not sure where and if extended or closed, but that at the very least shows the slide needs an adjustment. It also could be water intrusion has destroyed some of the bracing causing the misalignment which is common with slide outs.
