Rental Property
What is the fixed rate, what is the current variable, which bank. How many years left. Your ages.
What is the rent. Are you prepared to be a landlord.
How many years before it will be out of NE?
New Property
What term and rate of interest will you be paying your parents?
Do not discount getting a mortgage from a bank if you go with 100K and more from your parents. They might give you say 50% of 400K. I think that's preferable then borrowing everything off your parents.
Parents
Can they afford this, is the loan to both of you, will you be signing a legal agreement. Have your parents spoken to their solicior or accountant about this plan. Obviously if they are very wealthy and they can well afford it, then it's not a big deal.
Self employed
Well done on starting a business. Is it looking like a goer. Good that it's limited. Don't give personal guarantees.
Going Pear shaped.
You could move back into the rental. The new property has a buffer of 100K so there should not be an issue if you suddently had to sell it. Important for the parents. SCD is always going to be a good location.
Alternatively
Sell the rental, lose your 100K savings. Have to borrow 400K not 300K. Total borrowings on new home would be from the parents.