The thing I like to think about most is the space I have and how I want the plant to fill it. This means thinking about how large the plant will grow, when it flowers, what it’s sunlight and water needs are but also what shape the plant currently is and how I may need or like to shape it as it grows. If you have a particular shape in mind you need to look at a few different plants to make sure the one your buying is heading towards the shape you’re after. You then need to start shaping it as soon as you plant it.
You need to make sure you’re buying a healthy plant. Check the foliage. Make sure there’s no discolouring or mis-shaping of the leaves. Check the base. You don’t want to be able to see any roots and a lot of weeds in the potting mix is never taken as a good sign.
If you’re buying a flowering plant and there’s a question of the colour of the flower then you need to know what your buying will have the flower you want. However, you don’t want to buy a plant in full bloom if you can help it. Better to buy a plant getting ready to bloom.
Another thing I think about when buying plants is how many plants can I get out of the one pot? Often plants can be split so if you want a lot of plants is worth looking to see which ones you can break up.