SSL and basic website security for a rental site
A website address starting with https, shown with a padlock in the browser bar, is the minimum a visitor now expects before typing in any information at all.
SSL (the technology behind the padlock and the "https") encrypts the connection between a visitor's browser and your site, so that anything they type can't be read in transit. Browsers now actively warn visitors when a site doesn't have it — a warning that, fairly or not, makes an otherwise trustworthy small business site look risky at exactly the moment a visitor is deciding whether to trust it.
Almost every website builder and modern hosting provider now includes SSL automatically and at no extra cost; if yours still shows a plain "http" address or a "not secure" warning, check your hosting or builder's settings for a certificate option before assuming it requires a developer or extra budget — for most platforms today, it's a setting, not a project.
Beyond SSL, the two cheapest security habits for a small site are keeping your website platform and any plugins updated (most of the exploits that hit small sites target known, already-fixed issues in software nobody bothered to update) and using a real, unique password for your hosting and website accounts rather than one reused from somewhere else.
None of this requires technical expertise to maintain once it's set up correctly the first time — it requires checking, occasionally, that updates haven't been ignored for months.