Hasegawa - I have not built many of their kits, but I did really enjoy their 1/48 P-40E. Should cost between $25 and $40 I think.
Revell/Monogram - I am an old-school Monogram builder. I built all of their kits when I was a kid. In those days Revell was a separate company, and I tended to only get their 1/32 kits when I had saved enough for one or when it was Christmas or my birthday. If I had to pick a Monogram mold, I would go with their P-40B. Built it first when I was 3; built it most recently when I was 46 and many times in between. You should be able to get that one for less than $15.
As for original Revell kits, I would pick their 1/32 P-40E. Great detail in that kit, and a classic box art too.
Hmmm, I just said 3 different P-40 kits. I guess I am a fanboy!