If the bug really does bite I predict you’ll be wanting/needing to upgrade well within the year! I think you’ve been given some great advice already, but would add a couple of things:
Do as much learning as you can now so that when you are ready for the upgrade you are better prepared and have some idea of what to look for.
Buying second-hand is probably a good call, but you need to know what to look for. I would see if you can buy through a shop you trust, or a bike-specific platform like this one with seller ratings, but see if you can get a potential bike checked out by a shop or someone you trust. The sizing and basic specs is one thing, but it takes an experienced eye to spot things like suspension that could be problematic and lacking service support and spare parts, incompatible builds, etc. The seller may not be deliberately trying to pull a fast one, but might just not know any better, so don’t rush into a buy just because the seller seems honest. Many people will say a bike is in excellent condition if the wheels and cranks go round. You’ll want more than just that basic functionality.
A second-hand bike is an attractive option because it has already lost its value. So, assuming it was fairly priced, you could potentially resell it for the same as what you bought it for if you realise it is the wrong size or not appropriate. A new bike will lose a significant portion of its value as soon as it goes out the shop door.
Try to test ride as many bikes as you can. Friends’ bikes, demo bikes, whatever. Make notes on everything that stands out to you, positive or negative, from things like the sizing and geometry to the grips, saddle, handlebar width, etc. You’re unlikely to find a new one that has everything you like, but once you’ve worked out what’s important you can prioritise that and budget for changes to the small things.
Try to get an idea of what a bike will cost to maintain in the short, medium, and long term. Some parts, like the drivetrain components, wear out and will eventually need to be replaced. You can prolong this by looking after them, but the day will eventually come. Too many people have the idea that once they’ve paid the big initial purchase price, any future expenditure is just cheap basic maintenance. If you buy a bike with a Sram Transmission drivetrain, replacing that cassette and chain is going to cost more than a lot of the complete bikes in the classifieds, and you don’t really have a lot of choice for alternatives. But if you’ve got a 10-speed Shimano drivetrain there is a much wider array of replacement options and they’re a fraction of the cost. So, a bike purchase is not a once-off – you will be paying again and again and again over the years, sometimes out of necessity and sometimes for the joy of an upgrade or some bling. The choices you make in that initial purchase will determine how expensive those subsequent costs will be. A high-spec bike on clearance sale may be attractive now, but will you be just as happy with that choice three years down the line when you’ve had to fork out for an overhaul or two? Unfortunately this is something that is difficult without experience, but you can start by asking a few questions about a potential bike: 1. What are the consumable parts? 2. Are they available? 3. How much do they cost? You might be better off spending a bit more on the initial purchase to save on running maintenance costs or give yourself more upgrade options in the future.
Welcome to the sport, have fun, good luck, and drop in to ask more questions – you can see there are lots of others keen to help you!