If you have a Slimline SL5. sounds like your do, you actually have 5 LNB's. Three hide under the centered lens. If that is what you have, it matches your home setup. You sound good to go.
If you have an HR23, BBC's are not required. If not, you need one for each tuner, DVR's have two. They are little inline gizmos on the back of your receiver between the line in and it.
There is NO greater difficulty setting up an HD Slimline and an 18" round. I pay for HD so I want HD.
Read Edlanta's post. A plumb mast or level tripod, a dead-on shot at the 101, the elevation and tilt preset for your location, will get you signal every time. Don't believe the naysayers. Pay attention to detail.
Read the other posts here. This road is well rutted. Follow the advice of those who have been down it before.