namedy - You can't beat real wood! 4mm will be absolutely fine, it's enough for one sand should you want to do one, but you're unlikely to need to. Your builder is right about heels, though - they seriously wreck any wood floor. Although I think they'd make stringy vinyl mincemeat out of Amtico too. You'll just have to buy a load of pretty guest slippers for your party!! Or maybe some of those novelty huge ones with animal faces on the front. Quite a look with a designer dress. 
believe - Hooray!!! Your builder sounds good, I'm glad you've got him in your corner. What a relief, though. There's nothing worse than realising something is wrong at the end of the day when everyone has gone home and worrying about until the next day! I dunno about everyone else, but worries that are mouse-sized for me in the day become T-Rex sized at night.
titsy - Oak makes sense, it looks gorgeous!
extension - fingers crossed you get a bit of a reduction. I can understand the steel thing, they are expensive and they go up in hefty chunks with length (as I'm sure some of those on this thread with mahoosive ones will tell you).