My 2 cents. Most new equipment sold in the EU and in the USA is good enough for recreational diving. If it is dangerous, it's impossible to sell it, the local authorities would not allow it... It's just a matter of personal preference, for example if you feel comfortable with the mouthpiece, or similar. Then there could be the weight in case you plan to fly and bring your own equipment. And of course the price. If you buy new regulators you have to consider both the price of the product and the price of spare parts and regulators service that must be made regularly, some people say every year let's say anyway preferably not less frequently than every 2 or 3 years.
If you are into technical diving, it's a different ballgame, but then you would not be asking here…
Another thing to remember in case you do recreational diving is that some regulators are not suitable for diving in cold waters, but when I say cold I mean VERY COLD, let's say if you plan to do ice diving or lake diving in very cold waters like the ones in the alpine lakes in Europe. If you plan to dive in cold waters you have to check that the regulators you purchase are good for that too
If you buy second hand you can get very good bargains, but you need to be cautious provided several pieces of gear and regulators most of all require some knowledge and experience to be bought second hand. Anyway, they should be used like new and very recent, or relatively older but recently serviced, otherwise they could be not worth it. If you buy second hand without previous experience it would be good to buy only from persons you know and you can meet in person in your group or association