Hi and welcome!
It is great that you are doing your research first!
Could you please add your country, state or (for the UK) your county to your details. As we have got members from all over the world from very different climates and backgrounds, brands, access to vets and rescues, our advice can differ massively according to what is possible and available where you are.
In your case, it mostly hinges on whether you have got access to a good standard piggy savvy rescue. We have got a UK rescue locator on the top bar, but can provide a link to recommended rescues in other countries.
All guinea pigs in these rescues are fully healthy and have undergone a quarantine and any necessary medical treatment. Any sows have undergone a pregnancy watch to make sure that you won't have any unplanned surprises. These rescue piggies have also been carefully character matched, which is the key to a stable boar bond and to any happy piggy bond. If you have access to such a rescue, you do not have to worry about the gender (boys, girls or a sow/neutered boar pairing) and can concentrate on guinea pigs that are suitable for beginners, so you have got a very smooth start.
@sport_billy has given you our link in which we have listed the pros and cons of all possible combinations.
https://www.theguineapigforum.co.uk/threads/the-myth-about-boys.126564/
If you don't have access to a good rescue, then you have to weigh up whether you rather take the risk of potential mis-sexing and/or pregnancy or a fall-out between mismatched teenage boars. Boars need more cleaning of their bits, but sows often develop ovarian cysts when they get older, so it is not just one gender wins hands down! Rehoming from free-ads means that all the risks are on your side re. health/neglect, stability of the bond and an undisclosed pregnancy; desperate sellers can be more than economical with the truth!
An overactive grease gland can affect both genders although it is more frequently an issue with boars. You can stay on top of it without too many problems. It is also not something that happens constantly, but rather in spurts.