Bench width, seat depth, height and the clearances around it. The measurements that decide whether a bench is comfortable, and the ones that catch people out.
Cushions change how often a bench gets used more than almost anything else you can buy for it. What to look for in foam, fabric and covers, and how to stop them going mouldy.
A simple cedar bench is one of the most achievable DIY projects there is. Whether it is actually cheaper than buying depends almost entirely on what you already own.