It really depends on where you left off at and how much plat you got. Do you got an archwing? You'll want an archwing and a launcher so you can move quicker in open world areas.https://warframe.fandom.com/wiki/Orb...uncher_Segment
You'll also want to do the Deimos content for helminth and Necramech if you don't got them. Necramech can carry you through some content and is used for some missions.
https://warframe.fandom.com/wiki/Necramech
I'd buy one if you can just because its so much better than the junk ones they give you on missions that require em plus you wont need to grind as much then.
I think you might need a railjack for later missions as well which would also be a good purchase.
Demios stuff and operator amps are probably the most important things you need. A few important story quests require a railjack but other than that you can just join another players for any railjack content and while Veil breaker and Zariman add new content they don't unlock items/modes that will gate your progress in future content.