First, congrats on the new (to you) enlarger. Here's a couple answers for you. The 'big lens' is actually a condenser lens. My Beseler (which is strictly a condenser head enlarger) has two of them. They sit "round side up" and "round side down" ...sorta looks like a flying saucer. It sounds like you can switch between using this machine as a diffuser type enlarger and a condenser type enlarger. I have no idea how to do that... the manual or a call to Beseler could help with that.
Those brackets look like they might have gone under the enlarger board...check for pre-existing holes.
The other bit of info I have for you is to get some lysol wipes and clean the whole thing down inside & out. Since you have some fungus in the lenses, you might want to take them to a camera shop & see if they have a source who could clean them up, or give you a clue about it.
Hope this helps! Once everything gets 'squared away' you will have a great time with this!
Oh... one more piece of advice... If there is any way possible, try to get your enlarger off the floor. The dust that lives in carpeting will plague you like locusts!