I'm not sure AI really makes sense even for a small herd. I recall the cost ends up somewhere around $100/head. So for 20 cows that's around $2000 and as I  understand it you are looking at around a 60-70% preg rate unless you have a "cleanup" bull. So you still need a bull!

I haven't bought a bull for a few years however last yearling Hereford bull I bought from a very good breeder was under $4000.  I have kept and used a son and next summer a grandson from him and my best cows with excellent results so far.  Pick a breeder who raises bulls like you will use them and also believes in EPDs. I will only buy a bull from one of two midwestern breeders.

Go look at the bull and his sire and dam. Check out his disposition and that of his sire and dam and siblings. This is the way to choose a bull to build a herd imho and experience.