I think your customer needs to get some sort of a clue. Col's right, it's about learning the best way to get a car around a corner. Treat it ugly enough, and any car will understeer. My 25 was fairly unattractive, it wanted to be driven on the throttle, preferably whilst looking out of the side window. No attitude= no turning. Treat it right though, and it was quite quick.
I'd like to state here and now, there's no way you're going around corners on two wheels..
Tyre's= You get what you pay for. I'd be getting wider ones for a start.
Suspension= See above. King's are nice, monroes are fine. Custom are better, koni's/ Bilsteins rock..... keep going up in price and the handling becomes orgasmic.
I was told once by a wise man, understeer is when you're looking at what you're about to hit, and oversteer is when you aren't.