Whatever you choose to buy, be sure to also factor in the cost of an external drive or two for backup and storage. All of Apple's MacBook Pros come with SSDs now as the internal drive, and while one can get a 1 TB or 2 TB or more in the configuration, this can be very expensive. The thing is, with these laptops the internal drive and RAM are not user-replaceable, so the buyer has to know from the get-go exactly what configuration he or she needs.
Rumor has it that there will be a refresh of the iMac line this fall, possibly in October, so definitely a good idea to hold off on any purchase until it becomes clear whether or not this is happening. The current line of iMacs has retina screens rather than the older models' glossy screens. Baseline 21.5" iMac comes with a HDD as opposed to an SSD, and that hard drive is a 5400 rpm, meaning that it is slow. S-L-O-W. The iMacs can be configured with a 1 or 2 TB or more SDD. The current 21.5" iMacs are again not user-upgradeable in terms of the HDD and the RAM (although a store can replace the RAM). The 27" iMacs have a "drawer" that the user can open in order to replace RAM/add RAM. The current iMacs also come with the option of a fusion drive, which is a combination of an SSD (very small capacity) and a HDD (in the 21.5" iMac, again that same 5400 rpm, and in the larger 27" iMac, the HDD is 7200 rpm).
Using a MacBook Pro with or without an external monitor can work nicely for photography. I have a 2015 15" rMBP set up on a stand with external peripheral (Apple BT Magic Keyboard and Magic Mouse 2) and have been editing images on it for a while now. When I bought the machine I had every intention of getting an external monitor but at the time wasn't really processing many images so just never quite got around to it. Now I am contemplating the purchase of a new MBP and will continue with my same setup of the machine on a stand, etc., but am more seriously thinking about also getting an external monitor this time, as I have been getting back into photography and editing images more intensively again.
My current MBP has a 512 GB SSD, and this has been fine for me because I use several external drives, both SSD and HDD, for current storage, current backups and archival storage and backup. I have had a couple of iMacs in the past, but have found that I much prefer the flexibility of a laptop as opposed to a desktop, and that I can have the best of both worlds by setting the MBP on a stand and using external keyboard and mouse. As AFB mentioned above, an MBP can be put into "clamshell" mode, meaning that the machine can be connected to the external drive, set aside on the desk and closed, while the user has the external monitor in front of him or her and uses external BT keyboard and mouse. Or alternatively, as AFB also mentioned, one could choose a Mac Mini (if there will be a new version released this fall) and use it as a desktop with peripherals.
As for processor, regardless of the type of machine chosen: yes, i7 and 16 GB or more of RAM are preferable, especially if one is using any sort of editing program which is RAM-intensive or processor-intensive. Ideally, separate graphics as opposed to integrated graphics. Unfortunately, as mentioned, this is not going to be an inexpensive purchase, depending upon the configuration and type of machine chosen (desktop [iMac, Mac Mini] or laptop [MacBook Pro] and including in AppleCare and at least a couple of external drives or a NAS. If your wife is going to be using this for a photography business you may be able to get some tax breaks, though, if you're in the US.