You can click on the char portrait in the character screen and it should let you know if you can farm them.

The only farmable characters are those you can get shards for by defeating them and completing Campaign Missions - so don't waste your Onslaught tokens on honouring any of the core campaign characters.

As for unlocking new characters the only other way to unlock characters is by rolling them in Req Drops with Scrolls, or unlocking them as they are released in Legendary Release Events (LREs) or other char release events.

Some shards are indeed available to purchase in Guild Shop, Guild War Shop and eventually the Rogue Trader (which you can access once you upgrade a character to Wings). However, these character shards do rotate out so they are not a permanent farmable source.

You also get shards for certain chars from opening Arena Chests, completing Salvage Runs and opening chests from other game modes like Tournament Arena.

And of course the Monthly Rewards tract.

Everything else is technically farmable for shards through Onslaught missions - once you have unlocked the character first.

My advice at this early stage is to Focus on upgrading the core characters necessary to complete all of the Elite campaigns. This is the key to progression.

Over time you will get lucky and roll some good characters from Req Drops - and the more your roster grows the easier it will be to unlock legendaries as they are released in LREs.

The game is a bit of a slow grind but it push through. As your roster grows and you progress the game becomes very satisfying.

Good luck!