From my personal experience I have drawn up a table that shows how I suggest you spend your Plat depending on your game progression in Warframe. Following that table I get into more detail if you should spend Plat on warframes, weapons and (primed) mods.
|Early game (<100h)||Mid game (100-300h)||Late game (>300h)|
|weapon slots||weapon slots||weapon slots|
|warframe slots||warframe slots||warframe slots|
|archwing slots||archwing slots|
|orokin reactor||orokin reactor|
|orokin catalyst||orokin catalyst|
|(credit) booster||(credit) booster|
|mods (trade chat)||mods (trade chat)|
|occasional exilus adapter||occasional exilus adapter|
|occasional forma||forma three packs|
|prime parts (trade chat)|
Should you buy warframes for Plat?
I say no when it comes to normal (non-prime) warframes. The rather small grind to get those warframes is an important part of the game. Fighting harder bosses makes you effectively better at the game. Some warframes (including all Archwing warframes) can be acquired in the clan Tenno labratory with no grind required.
Prime warframes are different for me. Buying missing parts of a prime warframe via warframe.market or through the trade chat will shorten the grind significantly. That huge grind will not necessarly make you better at the game and there is a risk of burning out on Warframe while grinding for prime parts; it is just too repetetive.
Buying into Prime Access or Prime Vault warframes is something I think is okay if you have disposable income. I do not mean that you should buy into every Prime Access or Vault but there is no shame in doing so from time to time. Both Prime Access and Vault give you one pre-build Warframe, a warframe slot and some Platinum. You do not have to grind for the Prime parts, you do not have to grind for the ressources to build those parts and you do not have to buy one additional warframe slot. The largest Prime Access pack also gives you a 90 day credit and affinity booster. Buying into Prime will also keep the game development alive. Prime is definitely something for late game when you know you want to play Warframe for another hundred of hours.
Should you buy weapons for Plat?
You should not buy normal weapons for Plat. Weapon blueprints are widely available in the market, in all clan labs or as mission (and sortie) rewards. Ressources needed to build weapons from blueprints are in general more reasonable than for warframes. Before update 19 buying an Archwing weapon was something I would think okay because some parts were nearly impossible to get. But since update 19 all parts can be acquired through syndicate standing. If you really need parts and noone in your clan can supply those parts buy them from other players in the trade chat or via warframe.market.
Prime weapons parts in warframe.market or in trade chat are a good way to reduce the grind. If you buy into Prime Access or Prime Vault you will always get two pre-build weapons as well as two additional weapon slots. But weapons should not make you buy into Prime Access or Prime Vault. If you buy into Prime Access / Vault weapons should be the icing on top of an expensive Prime cake.
Should you by mods for Plat?
You should not buy mod packs the Warframe store but it is reasonable to buy the occasional mod from other players in the trade chat or via warframe.market. Like I said with weapons ask your fellow tennos in your clan first before starting an excessive spending spree. Do not buy too many mods. Keep focused on what you really need and what is impossible to get by normal drops (event mods). This leads to the next topic.
What about primed mods for Plat?
Primed mods is clearly a late game topic and you should have either a decent Platinum source or not shy away from buying Plat before you even consider to acquire them from other players.
So, back on topic: If you did not get a primed mod when Baro Ki’Teer was selling it fellow tennos will sell those mods for high prices. Some of those primed mods will take your frames and weapons to completely new levels of effectivenss. If you can reuse a prime mod on multiple warframes or multiple weapons it is worth considering to buy the mod. Keep in mind that buying a primed mod will set you back 1’000’000 credits. And ranking an unranked prime mod all the way up to rank 10 will set up back another 1’400’000 credits and 512 rare cores. Buying a fully ranked prime mod will “only” set up back 1’000’000 credits instead of 2’400’000 credits and 512 Rare Cores. Make up your mind if you want a great mod and grind for it for quite some time afterwards or a fully ranked great mod for an insane Plat price ready to use.2 People think it's helpful 😀