Buying off of KSL. I'd be pretty certain you'll start off with a 1/10 size. Put an alert on KSL for an 1/8 (the day after you get the 1/10) and that'll give you roughly 1-2 years to get that size, then you can sell the 1/10 if you'd like.
The biggest lie in musical instruments is that they appreciate in value. They don't. At least nothing that you're buying your kids. If you buy used you should be able to get out what you put in, assuming you didn't over pay. Never pay the "appraised value for trade in". It's inflated for the music shops.
Don't buy with plans of trade in, either. Those are super expensive as your size gets bigger. You should be paying somewhere between 50-60% of new. Easiest to understand are Eastman cellos. Go to their website and you'll see clearly the quality of the instruments based on the model number. In a 1/10 and 1/8 we didn't care as much about getting a higher quality instrument, but knowing the models will help a ton in understanding how much you should pay.
I own 1 of each size and will start selling the smaller models soon. Perhaps we'll talk again.
Where do you live? With our first we had a really hard time finding a teacher, but once we started it was easy to find another when we needed.
Happy to answer any questions