A few thoughts: Make sure that they do not use diplexers to piggyback the OTA and sat signal on the same cable - insist that they run at least 2 cables. The agreement between Voom and installers states that they will run up to 4 cables up to 125 feet. If you can swing it, get them to run 3 - otherwise you will have to run a third cable later to switch to the new receiver. Heck - 4 would be even better - you would have a spare in case one fails!
Secondly, Voom will upgrade you to a 24" or 30" dish or a better OTA antenna if the installer convinces them it is necessary. He would have to install the 18" and come back later with the bigger one (simple upgrade). A larger dish really is necessary so you don't lose signal in the rain. Some installers will go along with this, some may not.
Lastly, leave your receiver turned off overnight (in standby) for the first few nights so that the firmware upgrade occurs. It happens at 4AM. After the upgrade, the time will be correct and channel changing will be faster, in addition to other improvements.