For a Strength-focused start, aim for 2,000 Yen to buy Diving Impale and Mountain Splitter. In the shop configuration with Mountain Splitter at 10 STR and Fault Ripper at 15 STR, adding Fault Ripper raises the total to 5,500 Yen.
Diving Impale is the cheaper first purchase and gives you a forward thrust that staggers enemies. Mountain Splitter adds a heavy slash with knockback. Save for Fault Ripper after those purchases if you want another frontal sword attack and meet its requirement.
Check the Strength requirement separately from the price. Diving Impale requires 5 STR. Mountain Splitter and Fault Ripper require 10 and 15 STR in one shop configuration, or 20 and 55 STR in the other; follow the requirement in your current Skill Shop before allocating points. Having enough Yen alone does not unlock a skill.
Open the Skills menu to make your purchase, then check that the move is equipped. The skills guide covers the full requirement tables and equipment process.