i am a player in a session on foundry, the dm who is hosting doesnt have all the content i have on here, would i still be able to use my char content in his game or am i limited to his resources?

example: I just hit lvl 3 fighter and want to take echo knight. I own wildmount content he doesnt. would he have to buy it for me to get echo knight on his campaign?

There may be 3rd party extensions that might be able to help (such as Beyond20 browser extension) or maybe Foundry has homebrew tools you can use to recreate content. You'll need to check with Foundry.

It is worth noting that Foundry is not an official toolset of D&D 5th and are not licensed for selling digital versions of D&D 5th Edition content. It is a system agnostic VTT and any official content you want to use will need to be added in manually. So you can buy on D&D Beyond and then recreate on Foundry. However, what you as the player can make and what the DM will have to make is something I don't know about. 

When we play with dndbeyond on foundry, we use the character on dndbeyond and allow the beyond20 browser extension to report the relevant dice rolls to foundry. Thus foundry only really needs the actor's name to match our dndbeyond character name.

There's a Foundry module by MrPrimate called D&D Beyond Importer that can sync the Foundry character sheet with the ones in a campaign on here.

As long as they use the direct link for your character from D&D Beyond, anything on your character sheet will get imported.

The module has some advanced features for subscribers to MrPrimate's patreon, but that shouldn't be needed for the character importer.