Weed : 穷人我想这么做

Managed an empty lot for a friend. County warned about noxious weeds but told me "Our office uses Roundup Custom, 2,4-D Amine, and Vastlan for aquatic use." Their 3rd party service price is decent. I will kill one type of weed on my own but use their service for another weed. May need to spray multiple times over 2-3 years. 

I bought selective broadleave weed killers 2,4-D Amine (5 glallon). I am thinking to spray for lawn (not empty lot) at rental myself intead of asking lawn company to use fertilizer/weed killer mix (easily a few $100 per house per year). Unless it is weed everywhere, it may be not time consuming to spray spots on my own. Am I too cheap? 

Crab grass needs different weed killer, but not common on my rental houses.