1) It depends on the hardware of the server running, you can tweak the amount of simultaneous scans so you should always be safe in you tweak it down.
The biggest installation I know of has about 30.000 computers.
You can also ask the person from this thread : http://www.lansweeper.com/forum/yaf_postst2301_Big-network.aspx
2) Yes, you can select purchase order as a payment option