The spans on your steels are huge for residential, the cost of the supply of those is easy to find out. Hope you have worked out how to integrate (hide) them in the build or they will look ugly.
Although the builder is a mate he still wants to make money so do things formally or you will spend more than you want and have one less friend in a few months time.
Project creep is your enemy, make sure you and your wife agree on specifications and discuss your expectations.
You should have a big tabbed spreadsheet for all parts of the build so most costs can be accounted for. There will be surprises but not many if you plan well.
Check your dims, they differ in the 2 plans, might just be the diy drawings.
Done 3 large home extensions here (and £2 to £40 million ones at work), prepare for the dust and mess, particularly if you are planning on still living in the house. And a wife who will get fed up with it.
£80k might be ok but without a full bill of materials the cost of a door can be £350 or £2k, flooring £8/sq M or £60/sp M, your choice.
Good luck