All those modules are tested and compatible with the latest versions of PrestaShop 1.7.x. If you find other useful modules, include them in the comments. Here is our 2019 top 10 free PrestaShop modules list.
PrestaShop is very secure and vulnerabilities are very rare indeed, but here we are, recently a new security vulnerability has been found in PrestaShop modules.
When you are using free PrestaShop modules, be ware of this things may happen, and not every developer that offers "Free PrestaShop modules" has the good intentions about your website and you should double check and test your website and those free modules before using them in your live PrestaShop.
PayPal is very famous payment system used by over 267 million people world wide (statistics updated 1/1/2019). It is very easy to use and secure system to pay and accept payments.
I have seen a lot of PrestaShop stores, especially older stores that after a couple of upgrades, couple of moths of statistics, and the database becomes sluggish and overburdened.