It's hard to advise you Lewis. On the surface, what you are suggesting makes sense but you need to look further down the track. What will you be doing with this bike? Of course, we can't answer that and to be honest, YOU can't answer that. It's not unknown for someone to start road riding and get sucked in and go longer and faster and wind up with the latest uberbike from carbonframesareus.
Work out what you want to spend.
Work out what you can afford - this will be different.
Accept that neither of those figures are overly relevant - it's a given with bikes (being a consumer item) that you will always find something that will suit you better for more than you want to spend at the moment.
However, those first two numbers are useful.
Now go to a lot of bike shops (yes, long time viewers have heard this before but ...)
The more you look, the more bull you swallow from 'those trying to sell', the more bikes you ride, etc, the clearer your views will become.
On the surface, coming from a bmx would suggest a hybrid or other form of flat bar bike. However, (there is ALWAYS a 'but' or 'however'), what attracted you to bmx? Was it banging over the rough stuff? Was it pushing that bike as fast as you could? If it was pushing it as fast as you could over rough stuff, does that mean mtb now? No, it might mean a full on racer is the right move.
So, while you're looking at all these bikes in all these bike shops, try to get a feel for what you are attracted to. You may find that you immediately go to the mtb section every time, or the carbon section, or the section selling pannier bags, or the section with all the piccies of semiclad ladies.
I found it easy to work out where my priorities lie because I have a long history of road bikes, I have a long history of racing style bikes, I have a hybrid which I loathe, I love steel frames and ...
See, my history pretty much pushed me into one direction. I will always go check out the super expensive racing machinary, and I'll have an obligatory drool and will never knock back an offer of a test ride, but I know that I wanted an alrounder. I know all this because of the years I've spent mucking about with these flamin' machines and a long knowledge of what I do with them.
You don't have that luxury.
And so you are shooting in the dark to a large extent. You will be guessing with your choice and believe me, you will make mistakes (my beloved Europa is slightly oversized, but I still adore the old brute).
You can address this by getting as much experience as possible. Go to a lot of shops and make a flamin' nuisance of yourself. This has the double benefit of giving you the chance of discovering that wonderful shop that is ideal for you and so hard to find. It also fine tunes your !! BAN ME NOW FOR SWEARING !! meter and believe me, that is a good thing. But look at other riders on the road - can you see yourself riding a hybrid? Does your inner image tend to a full suspension mtb? Do you get all shivery when you see drop bars? Because you have suggested flat bar bikes, go to a bike hire shop and hire a flat bar bike for the afternoon - seriously, for a few bucks you might discover you hate the things. Sadly, you are unlikely to find a business hiring out racing bikes, but if you can talk your way into a test ride, DO IT. Never do what I did, decide that it's too much like hard work and walk away from a test ride - even if it isn't the bike for you, it adds to your data base.
And that's what I'm talking about, adding to your data base. The more experience you can cobble together, the better will be your decision.
Then it comes to cost. Unless you are very sure about what style of bike you want, it's probably best to think of this purchase as interim. My son, at 14, is dreadfully competitive, sports orientated and will go on to give me grey hairs when he learns to drive (the term 'hoon' springs to mind). There was little doubt that he was more suited to a racing bike than anything else and so I spent more than I should have to get him a good one.
However, if your direction is doubtful, being a bit more sensible is ... well, sensible. But who makes decisions on that basis.
Bikes are an amazingly personal thing. They go right to your innner dreams and images of yourself. They can be the ultimate expression of the man lurking inside you or they can be the ultimate camouflage. Whatever you buy, buy the bike you fall in love with. Seriously, I don't care how impractical it may appear. If you don't love it when you buy it, you never will. You do need to consider practicality, a carbon framed racer will not thank you for racing fire trails, but buying a 'sensible' flat bar bike is silly if you really want a racer (and that's not a put down of flat bar bikes by the way).
Take my circumstance. I love steel frames. I leap upon justification for owning a steel framed bike. I acknowledge the advantages of other frames but emotionally connect to steel. Similarly, I want a bike that is an allrounder, which is why my poor Europa suffered wearing mudguards and a dynamo for so many years. There are so many very good bikes that I would not have been happy with.
So, where are we in this rant, for a rant it has become
Take your time. Look lots. Ask lots of questions (and deliberately ask silly questions, you'll be amazed at some of the even sillier answers). And let your heart choose your bike. At your age, it's unlikely you have a lot of money. At your age, you will own many more bikes. Hell, at 17 you could honestly aspire to a full on mtb and a full on racer and a commuter to go to the shops. Maybe that is the answer.
Hope this helps. My god I hope this helps.