Erica arborea(tree heather) to buy or not to buy now

We've been looking for a small tree to fill a gap at the bottom of our 20m long garden. Really liked the tree heather Erica arborea shown in last week's episode and think it would do well there. It would be against a 5ft wood fence between two rhododendrons. We've found a 5ft version online (both in our 60's so need it to be 'instant') but not sure whether to get it now or wait till autumn. Any advice appreciated. Thanks. Linda and Graham