As Keith Bontrager said (and from the title of your post I guess you know!)
Strong
Light
Cheap
Pick 2
Having said that, £300 aint cheap. In terms of components, you’ll get fairly boutique for that. Hope hubs, DT rims (avoid Stans for heavier riders despite the temptation of weight), DT or Sapim DB spokes. Don’t be tempted by the bling of factory built. If he is heavy and rough on his components parts can be expensive and difficult to come by. Handuilts will be better suited to him, and easier to repair should things go wrong.
Who builds them is equally, if not more important. Find the best builder you can (I favour Harry Rowland, both for building and advice, although he is mostly into road wheels).
You don’t mention which discipline (XC, FR, DH?). Also consider 20mm or 15mm bolt through axel at the front and bolt through at the rear.