Sounds like you're going to be the 401(k) administer. Don't worry, it's not that bad. I'm also at a small company, and got to pick out the 401(k) plan, but that also meant I was in charge of running the thing. It's pretty easy.
Anyway, when I was looking a few years ago, the two top contenders were Employee Fiduciary and Vanguard (administered by Ascensus). Both of these providers offer plans that are better than probably 95%+ of 401ks out there. From what I recall, overall, factoring expense ratios as well as employers costs, Employee Fiduciary ended up being less expensive when the plan's assets under management were less than a million, while Vanguard was less once assets exceeded a million.