How do I buy new alloys? Anything I need to know?

I have a ford focus 2013 estate with 18" alloys and the tyres just wont hold pressure. I've had the valves changed, I've had the alloys refurbished (which helped for a bit but now they're back to needing pumped up every two or three days) and I've asked the local garage to look at it. Nothing works for more than a few months, so I've come to the conclusion that I need new alloys.

My question is simple - how do I buy them? What things can I get wrong or aren't obvious to someone who has never bought them before?

For example, wheelbasealloys lets me stick in the make, model, year and wheel size and then gives me a bunch of results. Will all of these be compatible? Will the required tyres be the same size as the ones on my car at the moment (can I re-use them or nah?)? Are there any safety issues with aftermarket wheels (BS standards to look for for example)? What's the "fitment" in basic terms and do I need to worry about it?

As you can probably tell, I'm not a car guy and I broadly don't really care what it looks like, I just want to be able to use my car without having to pump up tyres in the rain and dark this winter (and ideally not spend thousands doing it!).

Thanks in advance for any advice!