informal arrangements for some parts?

My husband and I are separating amicably and we were going to keep things informal initially.
He now wants to get a Separation Agreement so he doesn't have to pay stamp duty when he buys me out of the family home. He's suggested an online version but says we only need to fill in the box about him buying me out and can write not contested for all other areas. Is this sensible?
As I said, I am happy to keep things informal but if we don't write things into the separation agreement now does it mean they cannot be written into an agreement at a later stage?
We have 3 children.

Welcome to Wikivorce.

Separation Agreements are really just a "gentleman's agreement" written down, and either party is at liberty to break that agreement, and as its not legally binding, there's nothing the other party can do to enforce that agreement.

Only a Consent Order is legally binding and court-enforceable, a CO can only be written up as part of divorce proceedings, as it doesnt become enforceable until the Absolute is granted.

Some couples, who dont intend to divorce for some time to come (ie until 2 years separation has passed) have a separation agreement drawn up, which then forms the blueprint for the Consent Order. Additional clauses may be added to the CO, or clauses removed as the couple see fit.

The general advice is to not exchange monies, property etc. until you have a CO drawn up and signed.

Hope this helps.

Thank you.
If that's the case I'm happy to leave things for now but my confusion is that in some places I've read that they are 'legally binding' or enforcable by courts.

I have seen some advertised as legally binding which is misleading.

Correctly drafted and implemented a judge could accept the intention at the time if it becomes disputed, as Ruby said they are not legally binding though they are stronger than they were in the past.

Only a consent order is binding and enforceable, given they cost about the same i can only see sep agreements being of use to those who do not intend to divorce.