Yes, a clean install will definitely be better. You already know that it is good to have a dev version of Prestashop and test new modules there. Indeed, many of them may be damaged or incompatible.
It is better to choose paid modules from verified developers. You can also ask here on the forum about the developer or the functionality of the module before you buy it.
By spending a few dollars, you can lose several years of work in an e-shop.
Do not try to modify anything yourself if you do not have good knowledge of Prestashop and programming languages. It's really better to hire a certified programmer if problems arise.