I live on the Mainland of Shetland, was born here. I was away for six years, but moved back in 2006 and have been here ever since. A few things, in response to you and to some of the comments I've read. I haven't read the full thread because I don't have time. But didn't want to read and run.
Mainland Shetland has two towns, Lerwick (pop. 7000) and Scalloway (pop. 1100, and there is the large village of Brae which also has a population of 1100. Brae is close to Sullom Voe Oil Terminal and Gas Plant which accounts for its size.
Mainland Shetland's population is about 15500. There are three supermarkets, music venue, cinema, leisure centres, and various country halls. There are a lot of things for children to do, sports clubs, Guides, Scouts, music clubs, a whole variety of things. I do not recommend moving to an island any smaller than Shetland Mainland or Orkney Mainland. The smaller islands might have a shops, Unst is the most northerly of the Shetland islands, pop. of 650, a junior high school, primary schools, leisure centre, a couple of shops, but don't go to a small island like that expecting to find everything you might need or want. And then there are the tiny islands with less than 50 people and no shops. I think you need to be a certain type of person to cope with that, I certainly couldn't.
Ferries
The inter island ferries in Shetland are a nightmare, they are old and are constantly breaking and Shetland Islands Council (SIC) can't get staff like they used to to work on them. This has a huge impact on businesses in the small islands, especially mussel and salmon farming and fishing.
Getting off the islands is a challenge especially if you want a cabin and to book a car (in passing you need to be able to drive if you live in Shetland) on the ferry. You need to book quite far in advance and the Scottish government are forever making a mess of the ferries. Google it.
Schools
Your child is SEN? In that case you're going to find that there is support, BUT there a lot of SEN families needing support and you'll be at the end of a line. The support that there is thought is very good, and once your child reaches adulthood there are services run both by the SIC and charities which provide a range of opportunities. I don't recommend home-schooling here. There are a few families that do it but your child will be quite isolated, and you'll find that if you have your child in school the support you'll get is probably more than you'd find elsewhere in the UK. There are a few excellent ASN departments in a few of the primaries. And the same with high school. I work in SEBN.
Community
You will need to fling yourself into everything that you can, and you will find that you'll be welcomed. We need to make our own entertainment here and so community activity is really strong and there are many different sorts of things you could get involved in: music, sport, arts, hospitality, church. I'm on the committee of my local history group, and as a tour guide the Shetland Islands Tourist Guide Association. We need volunteers for everything.
Employment
Someone commented on your post that you won't be able to find work, or you child wouldn't and it's who you know. That's not true. Our unemployment rate is about 1% below the Scottish average. There is oceans of work available in Shetland. It's really hard for businesses to get staff, we want people to move here.
Land
I don't think you'd find it particularly easy to buy land. It very much depends how much you want and how much you're prepared to pay. But you would find that if you could buy some land and wanted to work it as a croft (which it very likely would be) you would find advice and support from the people around you. But be aware of trying to set up a 'new' business. You might have ideas that have been tried before and failed. Seek advice is my advice.
Winter
The winter is long and dark, so very dark. If it was snow and ice all the time I'd nearly prefer it. But it's the wind and rain, the wind really starts to get to me about February, the noise can be quite wearying. That type of weather is punctuated by a couple of days here and there of snow, which is like balm for the soul. The awful brownness of everything is made bright and cheery for a few days. I would recommend spending a bit of time here in the winter if you can. But that's not possible for everyone.
Summer
The summer makes up for winter! It really does. We've had a beautiful few days of weather, 13C degrees which is balmy for April. But the light, it's so bright and clear and when the sun shines there is nowhere like it, especially the evening light. It's lovely here right now. The sun is streaming in my window.
People
You will find nice people, I have a lovely book group with people in it from various places in Europe and Asia. You will trip over people from all across the world. I'm not entirely sure what draws them. Most of my best friends are 'incomers', which is a word I loathe. Very snobby, we're all just people.
I would say to the negative posters that they don't know you, nor your situation. You might move up, really love it, get thoroughly involved and be here for life. On the other hand you might hate it and not last six months. Only you know, and only you would know why you'd love it or why you'd hate it. But please do lots and lots of research. Mumsnet isn't the best place for that.
Feel free to DM me
I haven't proofread this, sorry for typos.