Is it possible to use Remote Desktop to control a Mac Lion Server without monitor and keyboard attached?
There's two parts to your question, one part is easy to answer: yes, you can get a remote, graphical session on your server without the server having to have a keyboard or mouse attached to it.

This part of your question troubles me:


  before buying the software


What software are you buying? Everything you need to remotely control your Lion server is included in OS X. From any OS X machine you can connect to any other OS X machine and get a desktop session on that machine.

To enable desktop access on a machine go to System Preferences -> Sharing and make sure the Screen Sharing setting is checked. You can configure just exactly who has remote access to the machine from this dialog as well:



With your screen shared on your server, from your Macbook you can open a Finder window and connect to the machine from there. First click on the machine name on the left bar in the Finder window and then click on the 'Share Screen' button on the top of the window to connect to the machine's display:



Depending on how you set up screen sharing you may be prompted to enter your user name and password.

The end result is a Apple Screen Sharing view of your remote machine's desktop:



(The computer you see up there is an iMac with no keyboard or mouse connected to it)

I recommend the neat ScreenShareMenulet program if you want quick access to screen shares. It sits in your menu bar and gives you one click access to remote screen shares with machines on your network.