For a first AR, it's hard to beat the entry level options from Smith & Wesson or Ruger for value, if you don't know yet what you like. If you have a little more to spend, it's hard to go wrong with a BCM 16" midlength. For an all-around AR, I am very partial to the 16" middy configuration, with an adjustable stock. Here's my first and so far only AR, a Rock River:
Mine just has regular handguards for now, but at some point I may transition to a free float tube like
this one. That way you can just put small rail segments where you want them, instead of having your entire rifle covered with cheese graters.
For a first AR, anything from 1:7 to 1:9 is fine. 1:7 will handle heavier bullets for long range shooting, but may over spin the lightest varmint bullets; 1:9 will handle the lighter bullets fine, but will be limited to 69gr or perhaps a little more on the high end. Most commercial plinking ammo is 55gr, which works fine in either twist rate.
As far as traditional DI gas vs. a forward mounted piston design, unless you are running an SBR or a sound suppressor, it seems to me that the piston doesn't really do much for you. A properly assembled DI rifle will run fine (mine is flawless so far, shooting a mix of brass case and cheap Tula steel case) and cleaning is a snap if you lube them well.