The 46LA is a decent engine for the Avistar, but you may consider a 46AX or even 55AX. Both of these have greater power which you will need when you try out aerobatics and ball bearing supproted crank which will give you better life. However, lots of people have flown the 46LA as well with good results if you are on a budget.

The prop I would suggest is the 11x6 or 11x7. Try it out and if you find it too fast after all adjustments, try a 12x5 or so. For a 55AX it would be 12x6 or 13x4. Remember a trainer needs speed to keep flying so I would keep the smaller size-higher pitch in each range. I would suggest Master Air Screw over APC as they have soft GRP that prevents prop breakage if you mistakenly hit the ground at take-off and landing.

Just use some soap water with cloth or a old toothbrush to keep the monocote clean. The toothbrush method, even if labour intensive, lets you detect breakages, cuts before they cost you the airplane. If it sags, use a heat gun set to low heat (I use a hair drier on my Phoenix as they come with cheaper PVC covering and a hair drier works quite well). Best still, borrow this from a fellow flier when you need it.

Keeping the wrenches and screwdriver handy is a good idea.

You dont need a electric starter for now, but get yourself a chicken stick. If your engine doesnt start, you can use a fellow flier's starter. Take the starter after you get a more advanced airplane

Look at what fuel you are using. I had a lot of problems till I was using plain castor. Using 100% synthetic is also not recommended as castor offers protection against wear when the lubrication is low. We now use Klotz 100 which has 20-80 blend of castor and synthetic in the lubricant itself. Other than this home brew is OK. Use a small amount of nitro 5-10% as it will give you more reliable idle. Nitro is its own oxidiser and will not let your engine stall if you suddenly apply throttle

And when you are up in the air, forget all this and simply have fun

Ameyam