If your Dad is interested in listening to local PD/FD on the UCAN trunk system, then yes, Re-banding would influence what you got him. I often get the question, "what is the right scanner for me to buy?". It depends very much on what you (he) wants to listen to. I've know folks who spend several hundreds of dollars for the latest and greatest, only to keep it tuned to the local Airport tower freq, or listen to the railroad dispatch. Do you know what your Dad might want to listen to? That will determine what scanner to get him.
I even once had a NASCAR (Non-Athletic Sport Centered Around Rednecks) fan ask me what scanner to buy, so that he could listen to the Cars-Pits comms from his living room. Not going to happen unless he lived at the track.
So bottom question, what do think he will want to listen to?
Right now the resale Web sites such as Amazon.com and eBay are beginning to have more and more scanners for sale as scanner hobbyists attempt to dump their old scanners while someone who is naive about rebanding and/or trunking will still buy them. We do not have a definite date as to when UCAN will "throw the switch" but it will likely be by the end of this year. Because of that, I would recommend that you purchase a scanner that is capable of being used AFTER the change takes place.
EDIT: I see gldavis beat me to the posting, but he is absolutely right. What your dad would like to listen to would be a big factor in selecting a scanner. If his interests are in the VHF/UHF range, then one of the scanners being placed on the resale Web sites might very well be sufficient. If it is public safety agencies, then I would recommend being very selective.
NASCAR (Non-Athletic Sport Centered Around Rednecks)
For Salt Lake City PD &FD you will not need a re-banding scanner. Almost any trunking scanner will work. For County, UHP and most of the other agencies in the area, you will need a newer scanner. Qlajlu has some good ideas on where to look.