Cassette for Turbo Trainer

The problem with getting a cassette that is too different from the one you already have is the chain length. Chances are, you're running a pretty long chain if you have an 11-34 block.

11-32 or 11-30 should be fine, but anything else and you'd probably end up with more slack chain than your derailleur cage can tension without actually removing links from it.

wouldnt worry on a turbo.  cheapest you can find thats compatible.  I have a wheel off trainer and run my climbing gears on that 11-32 they only get used oncenor twice a year so less messing around swapping.  Even a basic fluid trainer with no resistance i wouldn't worry about it.

Like for like means no set up issues or fettling needed with gears/derailleur etc. when swapping wheels

You can put on whatever range works for you, 11-23 if you want to, as above it doesn't matter so much on a turbo because it's you that chooses how difficult to make it not the terrain. I would however just check that this free wheel you got will take a cassette with the same number of sprockets as your existing bike. There are ways around it but if your bike is 11speed and the wheel is 10 speed you will struggle. 

I have a spare wheel that I use exclusively for the turbo. The cassette I have on that is a much tighter cassette than the one I have on my 'outside' wheels (12-25 vs 11-32). There are advantages and disadvantages to this.

To start with the main disadvantage - it means that riding indoors will feel even more different to outdoors. Your gear changes won't be comparable.

The main advantage I find is basically down to the fact that I don't need my indoor riding to feel the same as outdoors. For me they are different things. If I'm on the turbo indoors then i will be doing very specific sessions, with specific power targets and cadence ranges. Having a much tighter cassette makes this considerably easier. Outside I need to be able to deal with a much wider range of conditions and do not have such restrictive requirements on how I deal with those conditions (having the extra gear range is more important to me outside than the ability to maintain a tight cadence range across power outputs).

Ideally you would get the same cassette, so training is similar