I live here! I'm 32 and lived here all my life. It is not a busy and exciting place, we don't have great shops or bouncing nightlife, and the cinema offerings are rubbish (all common things people complain about). That said, I think it offers a great quality of life, for those who want to embrace it. The outdoors is amazing, its so beautiful with so many activities. There are clubs and groups for everything, so for those who have interests and want to, its easy to integrate.
For people used to having everything at their fingertips, Island living can be a challenge - our health service is limited so referrals off Island are frequent, there are no 24 hour supermarkets, the bus service isn't great, roadworks take forever and cost a fortune, etc. Things are also more expensive that you're used to - petrol just went up to 125.9p per litre today, road tax is higher, electricity is 16p a unit, groceries cost about 5% more in Tesco, etc. That said, income tax is lower - our higher rate band is 20%. Its a safe place to live and crime is low. We do have the unpleasant parts of society just like everywhere else - drugs, homelessness, domestic abuse, kids in care, etc. - but they're better hidden than say a UK city centre.
There is low unemployment. I know nothing about the building trade, but you would both likely need a work permit to work here, even on a self employed basis. The system is designed to protect and promote Isle of Man workers, except in exempt sectors, so you may want to look into the criteria for permits.
There's a group on Facebook called Isle of Man, what? where? when? how? & general help - it might be useful to ask about the building trade there. I often see posts from people thinking of moving here.
Any questions, just ask.
P.S. we don't call the UK the mainland; we are legally separate and we have our own laws, parliament are government... so this is the mainland
We call the UK "across".