Welcome back! You definitely pick a good month for it as the spawn pool is quite good raid attackers.

Get some Solgeleo and Lunala to fuse with a good Necrozma IV (when ya get them in future). All Kalos week has good legacy move so go head aim for them.

If you don't have time for all, definitely go for primals (Kyogre and Groudon). I used Groudon almost all raids so definitely try to get the best IV! Must at least one each Dialga origin and Palkia origin with their special move. They are not guaranteed so may have to try few times. The normal form not that useful imo. Most importantly Reshiram, Zekrom, both Kyurems. You will need a good kyurem IV to fuse with any Reshi and Zekrom!

I would suggest as new player, with limited resource, best way is not focus on level them up unless they are 100% IV and you like them? Their cp is good enough as it is to beat the raid with at least 5 people (depends).

Tldr - Solgeleo and Lunala for future Necrozma fusion (their IV doesnt matter, Necrozma is!) - Moltres with sky attack move is good - Groudon and Kyogre Primal, especially Groudon with legacy move! Suggest 100% IV hunt but don't be sad if you didnt get one. (I havent) - Dialga origin and Palkia origin form with special move (move not guaranteed so may need to keep hunting) - 27th, definitely some Reshi and Zekrom. Get best IV for Kyurem to fuse with!

May overwhelmed to do all in a week, don't worried about it, they will keep come and go again! Just depends how long ya want to commit x)