I'm going to be building my first AR15 and wondering if there are incompatabilities i should be aware of before i buy parts?

looking at getting a dpms lower with more then likely a different manufactures upper.  (from what i have read here the dpms uppers have a tight bore and the 5.56 rounds aren't really recomended in them.)

Primary use for the gun is varmit / home defense.

anyone with a good setup that wishes to share i'm all ears.

Quoted: I'm going to be building my first AR15 and wondering if there are incompatabilities i should be aware of before i buy parts?

Almost all your AR15 parts are interchangeable.... as designed.  Only things to be careful of are Colt parts, which may be bastardized.

looking at getting a dpms lower with more then likely a different manufactures upper.  (from what i have read here the dpms uppers have a tight bore and the 5.56 rounds aren't really recomended in them.)

This is no longer the case.  DPMS cuts chambers to NATO (5.56) spec, no longer to SAAMI (.223) spec (except on some match barrels).... so no worries.

Primary use for the gun is varmit / home defense. anyone with a good setup that wishes to share i'm all ears.

Need more info on the varmints.... and the distances you plan to shoot.... in order to make a recommendation.  Based on what you said, pretty much any AR will do the job, with an A3-flat-top upper receiver recommended.

prarie dogs, fox, coyote out to about 300 yards.

Then anything will do.  If you were going to be taking 600yd+ shots, then you might want to invest in a .5MOA shooter..... but for 300yds varmint shots (3" groups would be fine) then I would just get what suits you.  Might conside a 20" rifle length if a lot of your stuff is going to be long shots, for more velocity.  DPMS makes some nice, accurate barrels.

Thanks for the info!