Sometimes with secondhand laptops,the reinstalled Windows might be one of those copies with a key from a dodgy reseller,so you need to be wary. Also,you need to check which version too. If its an ex-business laptop,it might have Windows Enterprise installed on it over the shipped install,which is deployed as bulk image,and is licensed by the company per user. If this is the case,the install might deactivate itself.
However,luckily Dell laptops have a unique service tag,which can be entered on their website,and at least for the business laptops I have used,the original Windows image can be downloaded:
https://www.dell.com/support/home/uk/en/ukbsdt1
It will also give details of the original order and original shipped configuration.
Avoid Avast as they have been shown to sell user data:
https://www.cnet.com/news/antivirus-...browsing-data/
Avira is OK,and Kapersky and Bitdefender get some good reviews. You should also install an on-demand antispyware scanner too - malwarebytes and superantispyware are popular.
Also for Firefox and Chome there are certain extensions which should be installed if you want to maximise protection.Chrome is probably OK if you don't login with your Google account,but if you do it will definitely be able to put two and two together.
Examples:
1.)Privacy Badger/Adblock Plus/uBlock Origin
https://en.wikipedia.org/wiki/Privacy_Badger
https://en.wikipedia.org/wiki/Adblock_Plus
https://en.wikipedia.org/wiki/UBlock_Origin
These block cookies,adverts and other forms of tracking between websites - you need to choose one of them.
2.)Noscript
https://en.wikipedia.org/wiki/NoScript
Used to block scripts on the page.
Flashblock Plus for blocking flash content,but flash is not EOL so probably not really required now.