Yep, WordPress is a PHP application running about 30% of the top million sites on the internet so it is a big target for hackers and their bots.
It's also the thing that most shared web hosts offer their users in a "set it and forget it, you don't need to know anything about this" autoinstall process.
Those two things combine into an all-too-common story: people that really shouldn't be administering a PHP web application (or anything, really) are led into installing, configuring, and maintaining WordPress for their little food blog or whatever because it's easy to start with. Then they're left to the wolves as soon as the site is installed and vulnerable to exploitation.
This means that you will be miles and miles ahead of the majority of WordPress installations out there in terms of security if you implement anything vaguely resembling web application best practices. Even if you just make sure to update your site once a month! If you take a look at the vectors people get infected with, they are almost always vulnerabilities that were actually patched months (or even years) ago.
Example: The most infamous wave of infections to hit WordPress users in the past few years was caused by a critical vulnerability within RevSlider 4.14 and below, which was a slider plugin packaged with themes for sale on ThemeForest without the ability to auto-update itself. The devloper patched the vulnerability in February of 2014, but didn't tell anyone and didn't release the update to anyone besides their paying customers - so no one who got the plugin as part of a package deal had access to the security update. After a while, they released an updated version that people could use without having paid for their own license... but even after that, the majority of affected sites were maintained by people that didn't understand the importance of staying up to date and so it didn't help much.
As a result, this one plugin was the vector for a wide variety of malware campaigns throughout the next year... and bots are still roaming around trying to exploit this particular bug because there are still sites that are vulnerable to it.
So long story short, as a site visitor you should probably be wary of any WordPress site that seems to be slapped together with defaults for everything. My tell for this is basically: if I can plainly see that it's WordPress based on the URL, then I'll make sure NoScript is on and think twice about visiting it at all. The only exception is the wp-content directory because a ton of plugins require that you use that directory without renaming it. As a site administrator, you should make sure that you have a good security plugin configured and don't have a user named "admin" but otherwise you should be all set to avoid 99.9% of the WordPress "hackers" (script kiddies using WPScan) out there.
With that out of the way, regarding SEO:
According to this Yoast post, 301 redirects are the proper way to preserve link SEO when altering the link itself. In terms of images, they should still have the same media library URL unless you're changing the whole site URL as well... so you shouldn't need to create redirects for those.