In my opinion, try to find (or build!!!) something with at least the following:

* mid-length gas system - as battlemidget said, it will be better overall

* 1-8" or 1-7" twist rate -- 1-9" is going to most likely limit you on ammo due to lack of stabilization; I've never had a problem with lighter ammo (55gr) in 1-7 or 1-8 and they allow up to 75gr ammo. If you ever want to build a varminting rig, you could go 1-9 or 1-12 for lightweight bullets, but just build the upper.

* 5.56NATO or .223Wylde chamber. Stay away from .223Rem chamber as it limits you to .223Rem; 5.56 or .223Wylde will allow you to shoot both.

* Chrome-lined or not, up to you here in Colorado. But if you are wanting to do more "precision" shooting, CL won't be as helpful. In a "battle carbine", go CL for the wear and tear factor. I have both on various rifles for various reasons. A discussion on your planned use would clarify direction.

* trigger - Either an ALG mil-spec or something like a LaRue MBT -- the latter is a perfectly fine trigger for more precise shooting but still has a home on a run and gun rig.

Resale: it just ain't going to be good right now. If we enter another panic, you won't lose your ass regardless, but you also likely won't want to sell. I'd be hard pressed to go cheap, as it's unlikely you will build/buy and then decide you prefer to handicap yourself with a less efficient system.

Build vs buy: Build will require either you get some tools (which will be handy in perpetuity!) or borrow some. Buy will put you on the range immediately. If you build, you gain an intimate knowledge of function off the bat -- good for diagnosing any issues. Build allows you to customize off the bat. Buying cheap and then upgrading 2 months later is a route many go, but I think it's dumb and a waste of money in the long run.

Budget is necessary to know what direction to take, along with average/worst case scenario of use.