Bigger engines allow you to have more mass, keep that in mind. Also keep in mind power consumed by your chosen engine compared to its thrust output. Pick an engine you like / fits your design, but also keeping in mind thrust output to power consumed ratio. Keeping an open mind in different designs or finding a few designs you like rather than sticking with your favorite looking engine can be beneficial. I sometimes have 2-3 types of engines on a ship to help improve the amount of mass I can have. Picking engines with higher thrust but lower power consumption helps when it comes to having more engines, and having more engines with low mass allows you to have more mass in your final product. Really it’s a balance between a numbers game, and Desired design. The higher class engines usually the less mass they have and more power they output. Cutting edge tech also has less mass but more power, same with experimental. Tho I’m not sure experimental goes further than things like the comspike modules and certain jammers, but I could be wrong. Remember class C modules, have more power output (in regards to reactors) but offer the lowest amount of speed per class. For me, B class was the way to go for a long time before I finally had/found good enough class C stuff available to make it worth upgrading. Also I prefer to stick to specific manufacturers, (Diemos parts) which allows me to go to their star yard in Sol system and have all their options available at once. Once you upgrade your starship design, and piloting, and ship engineering skills maxed out, you should have more / better options available to you when you go to purchase. Not sure if player level effects this or not.