As TDQ suggests you need to pick your tyres carefully. I ran well-worn Goodyear F1s on my first Panda track day, and as it was wet, they were fine. In fact, I was a lot quicker than most - but many were struggling on "track tyres" and probably couldn't get heat in them. I ran Sportracs on the next day (and still do), it was cool, and took a few laps before any difference in grip was noticeable. They did wear through the day, but not to mush levels. Brakes were ok.
If you don't have any or limited track experience then the 100HP is fine. Yes, it'll understeer, but that makes it safer for a novice and it's a good way of learning how to get the most out of the car (i.e. diving into the corner at speed and then ploughing through with massive understeer is not!). If you're coming from proper race kit (see TDQ comments), then it would be frustrating but don't let that put you off. If you have never driven on track before, do this before entering competition and get instruction. You will not believe how much faster it is than road driving.