Never assume that all limousines are created equal. Not only should you investigate the size, make, model, and age, make sure you specify the color when you choose. If you assume that all a company's limos are white, you may be embarrassed when a black one shows up.
If you need a limo for a big event -- especially your wedding -- it's best if you make the arrangements 6-9 months before the event. Otherwise, you might be stuck with a limited selection of vehicles, or you may be forced to deal with less-reputable companies.