A Third-Party Administrator (TPA), which I assume is coming in to give your company the pitch, will charge for their services based on how many employees there are (among other things). For a company of only 3-5 people, I've heard of fees to the company as low as $1500, which frankly seems a bit low, up to $3600 or so, just for administrative costs. If the administrative cost to the company is of interest to you, then obviously you'll want to know what that is.
A Google search turns up the following from
http://www.401khelpcenter.com/faq/faq_45.html#.XRTDUOhKhPY:
"A Third Party Administrator (or TPA) is an organization that is hired by the 401k plan sponsor (your employer usually) to run many day-to-day aspects of your retirement plans. These include, but are not limited to, amending and restating plan documents; preparing employer and employee benefit statements; assisting in processing all types of distributions from the plan; preparing loan paperwork for plan participant; testing the plan each year to gauge its compliance with all IRS non-discrimination requirements as well as plan and participant contribution limits; allocation of employer contributions and forfeitures; calculating participant vested percentages; and, preparing annual returns and reports required by IRS, DOL or other government agencies."
In addition to low-cost index funds in general, you'll want to make sure there is a good smattering of total stock market index funds, total stock market "ex-US" fund or funds, something(s) that tilts toward value rather than a straight growth focus, a decent bond fund or three, and maybe a small-cap index as well. That would be a nicely full-featured list of good funds. Also, it could be a good idea to have the target date retirement funds. Even though those have slightly higher fees, they're good for people who won't rebalance their portfolios by themselves. Which is a lot of people.
Good luck!