I would agree with $50 given the amount you intend to invest. Between my wife and I, we have 4 LC accounts (2 IRAs), and have significantly more invested. I have moved up for some time now to $75 for the largest account.
I think you have to consider time required to invest such large amounts, too. If you invest in $25 notes and it takes you 3 months to get it all invested, then you have had several months with no return on a significant portion of your money. You could always invest at $50 to get it all invested, and then re-adjust to $25 if you find that the amount of (principle + interest) money that you receive is manageable at that level and that you are finding enough notes to keep close to fully-invested.
Keep in mind that you will always have sideline cash. I define that as money that hasn't been invested + money that has been invested in unapproved notes. A significant portion of that latter bucket will come back to when loans are not approved.
My accounts run between 2.6% and 3.5% sideline cash at all times, and I use the API and automatically fund loans 6 times per day. (Thanks for the off-hours tip, Fred.)