Can an iPad work to replace a computer in terms of photo management?

There are numerous hard drives that will work with the iPad. Some wireless, others via USB (with the camera connection kit). The lightening port is a whole other thing if you end up with the 4th gen iPad, though there might be a workaround.

Here's a 1TB wired solution from HyperDrive

Here's a 500gb wireless solution from Seagate

If you are seriously contemplating moving from a PC to an iPad just for photo management, I would recommend a 128GB or 256GB iPad Pro along with an iCloud Storage plan of 200GB for $2.99/month or 2TB for $9.99/month (USA Prices, other countries may differ) if you plan on adding tons of photos later, especially if you take tons of photos from your iPhones.

This will not only allow you to move over all of your photos successfully without having to worry about space and securing your photos. This also gives you plenty of storage for other PC and iPad related content such as files and documents along with apps, games, music, videos and books.

Even if you go for the 64GB version instead, always make sure that regardless of which size you pick, that you are backing up your iPad and that you have the photos saved on iCloud as well since anything could happen to the iPad.`