The $350 price is high but that's probably suggested retail, you can find better prices.  As for whether or not it's better to buy a complete rifle it just depends upon your situation and what you're wanting.

There are many choices for two stage or adjustable triggers.  RRA offers an assembled lower w/ their two stage trigger installed but I'm sure BFI would assemble one for you also (price ??).  Are you sure you wanna start w/ a target trigger for your first AR?

New lowers from the various manufactures are all small pin and will be compatible with most uppers except the large pin Colt uppers (not all Colt uppers are lg. pin though).

'How much' is determined by what you decide to go with.