A used aimpoint, the vortex units are nice, the primary arms units are decent and the Bushnell trs 25 I think is the model is s good right around $100 aimpoint type unit. 

Just remember the dot MOA size 2-4 is generally recommended (I prefer 2 but it depends on your eyes)*explained at the bottom

 the intensity/brightness for day use (most anything in that price range you won't need to worry about)   *if you are color blind you must physically look through the model as it seems each person in this category can/cannot pick up the dots in certain units.

Dot clarity should be good in this price range and you shouldn't have to worry too much unless you are buying some off brand import

the easiest way to explain the MOA is as, basically 1 inch at 100 yards - so a 2 MOA dot covers 2 inches at 100 yards 4 at 200 and so on so just be sure not to get an 8 or higher unless you are only planning on using it for 50 yards as 8 inches of target coverage leaves you wondering allot where on the target you actually are (if you can even see the target as the dot may be covering it completely )