starter rack or neutron or other semi modular

It's an interesting question to ask on a forum dedicated to one of your choices 🙂

Having said that, I would advise considering what it is that you want to achieve. For example, is it inexpensive fun, it is deep sound experimentation, is it something portable, is it something that can be battery-powered, is it something that can start small and grow with you, etc...?

Of course, in my opinion, the AE Modular is all of those things and more, so I was being naughty 🙂 Seriously, it's a fine I instrument that can tackle a lot, but it is different to the other things that you mention. You could have all of them and still get a lot from every one... and likely still think about more.

Therefore, in my opinion again, the choice is yours depending upon whether what you read and hear makes you believe that any of it will fulfil your needs. Understanding the niche that it will inhabit is key, and that can be 100% of your setup or just a small part of it. Personally, I use my AE Modular to sit and experiment in ways not open to me with any other pieces of equipment that I have available. I do have one semi-modular as well and it does very different things.

Possibly not the definitive answer that you wanted, but ask two people that question and you will get four answers.

I would say get AE Modular. With it you can get all the sounds other machines can (although might need to invest more in the future to get more modules). The only con comparing to other machines is that with AEM in order to get a sound you will need to make a patch with tiny cables meanwhile Crave, Neutron, Mother 32 - are semi modular and you can get a sound "straight outta box".

Myself I found a bit frustrated each time when I have a nice patch, but then need to get a different sound - have to unpatch everything - that's a con having a modular system (but not enough budget to get a few of everything).

I do really like the Neutron, excellent machine, and gives you a good range of possibilities. Like the AE Modular you'll really need a Midi keyboard, but it can be run  via CV/Gate no problem. Where the AE beats it completely is future expansion, you mention cost, but the same cost as a new Neutron will get you well started with the AE, then you can buy more modules as your budget allows. You do not have to buy one of the starter systems (though it is easier for getting started).

The area both the the Crave and Neutron lose against the AE is the semi-modular concept - the AE will be much more versatile. Having said that, the Crave I can see suiting people who want an electronic riff/rhythm/pulse/sequence, a bit like the Korg Monologue it's the sequencer that makes it special. To do what the Crave does with the AE will cost (a lot) more, but the AE will do so much more as well. You have to decide what your budget is, what you really want to achieve, how future proof you want the equipment to be and then pick the best solution.

Personally, having 2 of the 3 devices you mention, I would go AE not just because it is this forum, but you can't out grow a modular synth, and can spend as much as as little as you can afford - my AE has cost over £1,000 now, but it has been bought in several orders...