Newbuild advice please

After receiving two AIPs from different lenders I am considering buying a newbuild house. The estat agent is keen for me to reserve.

Firstly, is it normal for sellers to be flexible in terms of price and/or maybe paying something like stamp duty or some extra fittings?

Also, what will I be expected to pay prior in terms of solicitors fees etc? And, Amy advice about buying a house which hasn't been built yet.

Thanks