You can build a simple kart a lot more easily and inexpensively than a 250cc shifter kart. You can have a lot of fun with 5 hp, or you can get an 8 hp rototiller engine or some such at an auction, or you can pick them up for free sometimes. You could put a kart together in a week or two, and the education from building and driving it would make your second go kart twice as good as it would have been otherwise.
There's no rule saying that you have to have a "baby" go kart before you graduate to a super cool one. But it's a fact that you have to do a lot better job of building the high powered one because of the torque, weight, etc. The value in building two karts is in the education and experience.
For example, I built my first mini bike this spring. I thought it was a pretty smart design. But I learned a few things in the process. I learned that my property where we will do all of our riding has special requirements for suspension and the types of tires we should use. Also, I learned just how much power is required to get good performance going up the hill on my driveway. I could make an educated guess, but it's nice to know for sure.
Anyway, after my 2.5hp Harbor Freight powered dinky kart and my first mini bike, I feel like I'm more competent now to build a Honda 200 shifter kart and a bike with spring suspension. I've already sold the bike to a college kid who plans to ride it to class, and I'll find some kid who will enjoy the kart now that my kids are ready for something bigger.