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Recommended designs can greatly vary depending on what you're trying to play (singleplayer/multiplayer) and the objective. It also depends on what version you're playing since you can change the version rules which is like switching between patches for the game. Weapons that are considered powerful in one version can be average at best in another one, so these all factor into your choices.
The best advice to heed from the start is to avoid trying to buy everything in the shop, and just focus on purchasing the parts you want most to complete the singleplayer. Upon completing the singleplayer story twice, once with each main story path, you will automatically receive every single shop part in the game. Below is the game's mission tree, you want to complete the game with the League path and the ORCA path. Most missions are on the short side, so don't let the mission tree confuse you.

When fighting enemy ACs, I usually just equipped dual Gatling canons and hosed everything down before they killed me. So try this tactic if you find yourself stuck on a mission where you must fight two enemy ACs on your own. Otherwise just have fun, design an AC that suits your playing style and that you enjoy using, and try to get as much FRS memory and bonus weapons as possible by beating missions on hard mode and getting S ranks. Let me know when you get it, I'd love to play some online matches with you and trade some designs.