[duo797]

The verb いる as in 'to need' has a kanji. I'm not sure about the usage but I've seen it used by a variety of sources. As an aside, the kanji for 要る(to need) is also part of the compound 必要（ひつよう） which also means 'need' or 'necessary'. When I check a couple of places, all the example sentences use が　(in some cases, は, but you were trying to distinguish が vs. に). Also, I'm certainly not a native speaker, but something feels a bit odd about using に in お金に要る。

About しなければならない and all variations thereof, don't make the same mistake I did when I first learned it. It means 'have to' or 'must' do something, but you don't use it in all the same places we say 'have to' in english. Correct situations you would use なければならない, 'I have to turn in this paper', 'I have to do my homework' etc. I'm making this point because I used it incorrectly to mean things like 'I still have to watch this TV show' or 'You have to read this! It's so ridiculous!'. You'll figure out more what I mean through using it and seeing it used properly.