What to buy with 1000 platinum if you are a new player?

I'd get a bunch of cosmetics and save a bunch for Warframe and weapon slots.

Being able to customize your gear makes it a lot more fun to use IMO. Since you can farm for functional items like Warframes, weapons, mods, etc., I find it nicer to just follow the progression of the game.

I recommend checking out Color Pack: Alpha, Color Pack: Beta, (maybe the Kuva Lich Hunter Collection), the Shi armor set, the Sigma Series armor set + syandana. You can also just have a look up armor sets and syandanas in the Market to see what's available.