Have you driven one of the animal karts yet? I’d recommend going to a practice day at your local karting club and seeing if someone will let you have a go in their kart before deciding. I posted a thread very similar to this about a year ago but after trying out some karts I decided to start with something a bit more manageable than a shifter because what I really wanted to do was to be able to race often and against lots of other people and because coming from cars I found even the 4-stroke kart to be a handful. In my area at least, CR125 is only popular on the road track races which don’t happen all that often, and the most popular sprint track class is World Formula, which is similar to the animals in your area. More than anything else I think having lots of people to race against is most important for a first kart.
The 4-stroke karts aren’t all that fast but they can make up to 2.5 lateral Gs in the corners so they’re still exciting, especially when combined with the fact that you’re in a giant pack of people inches from one another. Now that I’ve got a year’s racing experience I’m looking to get a shifter in particular for road racing but I’m definitely glad I started with the cheaper, slower class that enabled me to go racing with a huge starting grid almost every other week all year on a relatively small budget.
Plus, a $3-3.5k startup budget can buy a front-running used animal package that you can win races with when your skills catch up to the kart or a dog of a CR125 that you will be struggling with constantly. Remember to also budget for some gear, tools, and towing equipment if you don’t have it already. Also, most chassis work with a shifter engine as well as the animal, if you choose a pretty modern chassis probably all you need is to stick front brakes on it when you’re ready to move to a CR125. Alternatively: buy a shifter chassis as a roller, remove or disconnect the front brakes, and run the animal for a while on it first while so you can get lots of seat time while you save up for a nice CR125 engine.
If you want to see what 4-stroke racing is like I’ve got a youtube channel where I’ve been uploading some of the races I went to. I took it to both road and sprint tracks: http://youtu.be/116b-mJBer8