Depends on how you do your photography work. If you travel and want to work on your images while on the road, the MBP is your best bet. If you bring your images home to work on then the iMac displays are much larger and quite nice. As
@Zxxv points out, a MBP with a monitor can give you the best of both worlds depending on what you're willing to spend.
The iPad Pro isn't a viable alternative for photography work. I can use my iPP for most of my casual computing needs, but I haven't gotten comfortable with it for photography yet. Affinity Photo is available for the iPad though and worth checking out.
The other point I'll mention because you mention that you're pensioner-- bigger displays can be much easier on the eyes.
You'll want external storage for two reasons: bulk storage and TimeMachine backups. SSDs are fast, but expensive, and you want enough free space on the drive that it doesn't slow down too much. Keeping the bulk of your library on an external drive and the images you're working with on the SSD with is probably the best approach. Time Machine is a must. You'll want your backup drive to be separate from your photo library drive, and the backup drive should be big enough to back up your local hard drive and your photo library drive.