I always recommend you start by renting, even if you can afford to buy a harp outright.
First, you want to make sure it is, in fact, something you (or your child) will enjoy playing and want to stick with long term. It’s easier to return a rental after losing interest in 3 months than to sell a harp.
I also recommend starting with renting because you do not yet know what type of harp is going to be best for you. Should you get a pedal or lever harp? How many strings? How tall? What finish? Do you need a certain weight? Type of wood? If lever harp, how many levers and what kind?
Additionally, people who are not harpists do not know which brands to be wary of or how to tell if a used harp is in good condition or not. Getting a harp teacher involved to help guide you through this process and help you find the perfect match for you is worth the wait.
It is always best to try a harp before you buy, whether used or new. Even harps that look identical, same make and model, can sound different due to the variances in wood. You may prefer a bright, vibrant sound while someone else prefers a more mellow, even tone. However, unless you have the funds to travel, happen to live near a harp showroom (the picture at the top of this post is a wonderful showroom that used to be in Southern California but is now run online), or find someone locally selling the ideal harp, you will likely need to consider an online purchase. I have purchased some incredible harps from places like eBay, Craigslist, and other classified sites. But I know what questions to ask, pictures to request, and things to watch out for. I also know there is still a risk I am taking buying sight unseen.
One more note. If you decide you want to buy a harp that is more than you can afford, you may be able to get financing through Allegro credit, which specialized in financing musical instrument purchases. Many harp manufacturers and dealers offer financing through Allegro. Talk to your harp teacher about this option.
In summary, never buy a harp without talking to a harpist about it first. Find a teacher, see if they can connect you with a harp to rent for the first few months, and start talking to the teacher about what you want in a harp. You will be so much happier with what you end up buying.