The SIG LT 40 is an excellent choice if you want to build. For a first plane though, I strongly suggest an ARF.

There are several reasons for this:
1. Learning to fly is difficult enough without adding in the possibility something may have been built incorrectly (or have a warp in it).
2. When you build, you tend to become emotionally attached to the plane. In a trainer this is not a good thing as a trainer is expected to get banged up. If you are emotionally attached, it will inhibit your ability to learn because you don't want to see "your baby" hurt.
3. There are several ARFs out there (Hobbico SuperStar and Avistar among others) that cost about $100 already built. You will not be able to purchase the kit and the necessary materials, not to mention your time to finish it for that amount of money.

While the RTF planes look like a good deal, in the long run they are not.
The engine provided, while fine for a trainer, USUALLY will have only marginal power for use in a second plane.
The radio is a BASE 4 channel radio that USUALLY has mechanical servo reversing switches. The mechanical servo reversing is OK, but the switches tend to mess up after a couple of months of use because of dirt, grime etc that gets in them. In addition, USUALLY by your 4th plane you will be wanting flaps and/or retractable landing gear - a 4 channel radio can not feasibily do this.

I suggest, getting a trainer ARF.
A decent ball bearing 46 engine (Thunder Tiger, GMS, OS FX/AX). The brands I mentioned are user friendly, very powerful and highly suited for use in other planes. I suggest staying away from the OS LA series and MDS engines.
A 6 channel COMPUTER radio (or more if the budget allows). The additional channels and features of a 6 channel computer radio will ensure you will not outgrow the radio for several years.

If you do decide you want an RTF, I CAN NOT RECOMMEND the NexStar. There have been too many problems with the AFS. Most users turn it off. Now you are paying a premium price for something you can not use.