I also would suggest you give some consideration to paying additional on the loans probably starting with the one with the highest interest rate. That is like a sure 6.8% on your money which at this point isn't a bad return.
If you insist on investing I would suggest a total stock market fund in a taxable account. With Vanguard this fund has a $3000 minimum I believe. If this gives you a too high allocation to equities in your total investments you might consider changing the Roth from the 2050 fund to something like the 2030 fund.

Bob