I don't know the current market, so no suggestions on bike models to fit your specs. However, I have run a rental fleet (40-50 bikes), so here are a few suggestions:
1) Buy quality up front and you will reap savings on maintenance and repairs.
2) Think carefully about the range of sizes you will want.
3) Simpler bikes require less maintenance. If you really have flat terrain and low winds, single-speeds are great; otherwise look for an internally-geared hub.
4) Think about having loaner locks and helmets, since your customers won't bring those.
5) Kickstands are essential, chainguards pretty important, fenders and baskets are nice.