A great premium plugin requires a flawless theme. Since you're reading this, you are probably not the one to gladly embrace a WordPress standard theme. The ever-so crowded free repository contains a couple of gems. As millions of people have found out. So unlike all those other cheapskates, you are very willing to compensate a professional coder for his efforts.
What could possibly go wrong? Unfortunately, the vast majority of premium (and free) themes is outdated, full of bugs, not adhering to WordPress best coding practices or simply way too unflexible. (Hey, I'm all up for digging in the source code. If it is well structured and commented.) The best and most popular themes these days are rather frameworks than single-purpose templates. They feature drag-and-drop layout editors, come in all responsive flavors and make your site pass the Google speed test with ease.
If you are not sure wether your current theme is up the challenge, I suggest you go with one of these options. I tried and tested them in many of my lectures – ultimately, the decision is a question of taste. Mine is very simple like Orson Wells'.