An OTTB can absolutely be fine for your first horse. The key is finding the right OTTB. There are some that are super spicy, very green, not very trainable. Then there are others, like the one I bought for my second horse, that are quiet, sensible and very, very trainable. There’s also everything in between.
You want one of the quiet, sensible, trainable ones.
Mine is 10; he raced (poorly) until he was 5, and then spent a lot of time turned out and/or being ridden inconsistently and probably not all that well. When he showed up at my trainer’s, he needed groceries and training, but he has a fabulous brain, and he is really well built. He takes good care of himself (sensible), he’s a real thinker (trainable) and while he’s forward off the leg, he’s not at all hot. I’d put my 85-year-old grandmother on him for a walk without a second thought.
Do some research into pedigrees and which TBs are known to pass along good brains and trainability. Be realistic about your goals. IMO, having bought the wrong first horse, having a good experience with your first horse is really important. I had a not-so-good experience with mine, and I ended up taking an almost five-year break from horses after finally selling her. On paper, we were a decent match, but she had zero work ethic, wasn’t very trainable, and was definitely not sensible. I could ride her, but it wasn’t anything like an enjoyable experience.
What I love about OTTBs is they almost always come with a work ethic and a lot of life experience. It’s not the same type of experience that they’d have in a H/J barn, but they know what trailering is all about, they’ve been worked around other horses, they understand that they have a job to do.
I wholeheartedly concur with the posters above who’ve suggested getting the assistance of someone who is good at seeing a potential sport prospect OTTB and can find one that is a good fit for you, both in terms of what you are ready for and what you prefer in a horse.