I have these sentences from my textbook which I mostly understand, but I'm confused about the usage of 成.
”有银行通知我去面试，所以就穿成这样了。“
”我们和上次那个公司的生意终于谈成了。“
"当别人请你帮忙时，对那些自己很难办成的事情，最好还是先想清楚。如果最后事情没办成，不仅自己会觉得不好意思，而且别人以后也有可能不再相信你了。"
I'm guessing 成 is a resultant complement, it seems from context to mean something like 完 or 好, indicating completion. If so, how does it differ from them?
As a resultant complement, both 完 and 好 can indicate 'finish' or 'complete'
Example:
谈完生意 (finish talking business)
做好功課 (finish doing homework)
~
好 can also mean 'properly' or 'well'
Example: 學好中文 (learn Chinese well)
~
As a resultant complement, depending on the context, 成 can mean:
成為 - e.g. 做成木偶 (make into a puppet); 打成殘廢 (beat to become crippled)
成功 - e.g. 谈成生意 (successfully making a deal); 練成武功 (successfully trained in martial art)
很难办成的事情
很难办成 = hard to successfully do
穿成这样
穿成这样 = dress (to become) like this
In the last 2 examples you are right , but in the first one 成 means 成为(become) which can not be replaced by 完 or 好.
