Welcome to the forum, and I hope your experiences with Arduino are as happy as mine have been! :-)The Atmega328 is pretty powerful, but you need to be a bit more specific about "sensors". Are they analog, digital, or what?
You can make your own board, sure.If you use an ATMega328P it will have exactly the same inputs/outputs as a standard Arduino. No more, no less.
You can get analog multiplexer chips. For weather data you won't need thousands of measurements a second.http://www.gammon.com.au/forum/?id=11976
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