The Avistar is an excellent plane. It's semi symetrical airfoil will help you with aerobatics. I would suggest staying with the Avistar as you are already familiar with it.

In case your father-in-law has not mentioned it, I would suggest NOT GETTING AN RTF (Ready To Fly).

The engines in the MAJORITY of RTFs are bushed bearing 40s. While theyare adequate for a trainer flying off of pavement , in grass (especially if it is a little long) the power will be marginal. In addition, the bushed bearing engines are ALMOST NEVER SUITABLE for use in a second plane.

Again, MOST RTFs come packaged with BASE LEVEL 4 channel radio. The radio is frequently on one of the more heavily used channels and may or may not be compatible with the brand of radios commonly in use in your area.

I suggest the following:

Get the Avistar plane.

Get a decent 46 ball bearing engine. OS FX/AX series, Thunder Tiger PRO series, Evolution brand engines are quite good and VERY user friendly (99.9% will run fine right out of the box).
Use and 11x5, 11x6 or 11x7 prop with the 46 engines. Any of these engine will have pleanty of power for your trainer AND for your 2nd, 3rd, etc planes also. There are othe very good engines out there, but these brands I have experience with and can recommend. If you are considering a MegaTech (made by MDS) or MDS brand of engine, do a search on MDS and form your own opinions.

Get the brand of radio MOST COMMONLY used in your area. Also find out which channels are the LEAST USED channels at your flying site and get the radio on one of the least used channels. This will maximize your flying time and minimize your chances of being accidently "shot down". If your budget can afford it, I highly suggest a 6 channel (or more) COMPUTER radio. Just starting out, you will be using very few of the additional functions/channels. By the time you get to your 3rd or 4th plane you will most likely be wanting items like flaps or retracts, plus many of the additional you will also find handy.

Don't be in a rush to get to your next plane. When you can fly the Avistar well, convert it to a tail dragger and learn to fly it again. When the plane can no longer teach you anything, THEN it is time to go to your next plane (usually a low wing, something like a SIG 4*, World Models Rambler or GP Sportster). At that point all you will need to learn is the new things that plane can teach you. Many people move to a new plane too soon and get in trouble because thay have not learned enough from the trainer.

Chances are your father-in-law has already covered these things, but in case he hasn't.