How does soil type change the tractor HP I should buy?

Yes, soil is a big factor. Heavy black cotton soil sticks, holds water and resists the plough, so it needs noticeably more pulling power than light sandy loam for the same implement and depth. A farmer on light soil might manage a job at 40 HP that needs 50 HP on sticky clay. So for the same acreage, plan higher HP on heavy or wet soils and you can stay lower on light, well drained soil. Also think about depth, deep ploughing and subsoiling pull far harder than shallow tillage. Tell the dealer your soil type and your deepest typical operation and ask them to suggest the HP band, then cross check against the HP range printed on your main implement.