here's my 2 cents. a.i. is a very good option for someone w/just a few cows, BUT something that hasn't been mentioned, you will need some pens w/at least a head gate to catch them & hold them in to be a.i.'d. also, with your schedule the best option probably is to sync them. even w/synchronizing, you still get best results from watching for a standing heat after sync. your conception rates will be slightly lower w/synchronizing than with natural heats so it's very likely you won't get 100% bred.
so, i would suggest for a part-time owner (i mean one that works off-farm) is to buy a bull. it doesn't have to be the highest $$ animal that you find. follow bill's advice & look for good conformation, temperament, and have him semen tested. many sellers will pay for this themselves. if they don't and you have it done, make sure when you buy him that you have him tested before you put him out w/the cows and have an agreement with the seller that if he doesn't test, they will replace him or refund him. that's the problem w/oregonian's suggestion......bulls that go through the salebarn are usually not tested or guaranteed & for a newbie there could be other problems that are hard to detect w/o experience. you should be able to find a young bull in the price range you are looking at and for just 4-5 cows, a bull about 15 months should be able to service them if he's big enough (usually the younger bulls are a little cheaper)