Regardless of F2P or spender, you're aiming to save for rebates. Focus on saving for Fukurokumaru's Deals (a coupon rebate) and getting a viable position one (main damager). Common affordable position ones are Obito Uchiha [Rage Mode] four star for breakthrough and Mecha-Naruto four star for kit upgrade.

Mains, including water main, are supports, so you're less likely to build around them and simply adjust their talents to help your team.

Common starter team is Choji GNW, Hashi NY, and Guy / Tsunade Sannin.

If you want quicker help, I recommend joining the official discord: https://discord.gg/narutoen