Battery charge light on the dash? Or perhaps you're having to constantly recharge. Chances are it's the alternator at fault. Don't risk 2nd hand units when you can get brand new items at this price! Quality aftermarket alternators for a direct fit.
The alternators can be identified by the shape of their pulley. Mk 1.6 models use a 'V-Belt', meaning there won't be any ribs on the pulley, instead it's a trough the entire way round. Mk1 1.8's use a 4pk belt, meaning a wider, flat belt with 4 ribs.