I'm pretty sure he won't ruin the country. If he can't come up with plausible, feasible plans for how to run the country, his MPs aren't going to allow him to enact those plans. He'd have to win a pretty convincing majority, with the appointment of a lot of new Left-wing MPs, to be able to do anything he wants. Anything short of that and he'll need a watertight plan that's backed by all groups in the PLP.
My point is that if he gets into power with the backing to enact his plans then he will ruin the country. If he gets in as part of a coalition or without the support he requires then he will just get shut down in every area that makes him unique and will just become another politician accused of failing to enact all of their election policies.
Most of his ideas are plausible and feasible but they will have negative consequences when the victims of his plans fight back or leave.
National wage limit - you can't do that, that would genuinely lead to a huge number of people leaving or as it is now, they would just find ways around it.
Stop tax avoidance/evasion. We would already have this problem solved if it was possible. My dad works in tax and it riles him no end but you have the government embroiled in these cases for years and years and its the government lawyers vs billion pound companies and the best lawyers in the world. You shut down one loophole and another is already ready to be used.
Stop companies exploiting the loopholes of international tax law and you may drive them out of the country and lose jobs and the tax they do pay.
Education - another aspect of society that is apparently easy to fix. We have a system that allows children to learn. Is it the education system that lets children down or is it their upbringing that places no merit on learning and is in direct conflict with the school. Most underperforming schools are in impoverished areas and the parents are 90% of the problem. "I didn't learn at school and I'm alright" mentality.
Renationalising energy - how do we take back control over these without either paying a kings ransom or essentially telling companies that you can invest in the UK but we might decide that you are making too much and we are just going to take it back when it suits us.
Most of his ideas are nice but impractical and have severe consequences if they were actually implemented.