First, I would say that using the radio in your existing plane is a great idea.

Next, getting the Mini 3-D is totally dependant on your skills, and how you intend to fly it.

If you set mild controls, you may find it an easy flier. On the other hand, you may break it the first time up. Let's face it, everyone is different. Two people with the same plane and the same stick-time may have two different results.

It's like the person who says "I've been driving for 30 years and never had an accident". Does that mean they're a good driver, or just lucky?

I would say a very good option would be to build a 20 size SPAD that is similar to the Mini. Fly it around, bash it up, and get used to it's characteristics. If you can easily handle it the first time out, put the engine and radio in the Mini. And you're only out the 10 or 20 bucks you spent on the coroplast. However, if you find it's very difficult compared to your trainer, you can keep fly/bashing it until you've gotten the hang of it.